I went to another lecture at my local college, and this is what came out of it. Not really connected to the first Universe oneshot.

There are many things about the cosmos that no one understands, John. Not even the most intelligent astrophysicists nor the most dedicated cosmologists know exactly everything about the universe because we are a microscopic blotch on the fabric of space-time. If humans, if Earth itself never existed, nothing would change. The galaxies would still move away from each other, the stars would be born and die in beautiful supernovae, the dust that makes every physical thing would continue its journey through the incarnations of planets, the virtual particles would still fly unseen through the vacuum of space. None of this is changeable, and that is why I'm writing to you now.

John, there is a massless, invisible, without matter, something. And that something will change us. You and I, John and Sherlock.

This something is called dark energy, but it takes a bit more explaining than that.

Early on, perhaps the 1920s, a man named Edwin Hubble discovered how far galaxies were away from us by analyzing the lumosity of Cepheid variable stars, stars that grow brighter and dimmer in a regular pattern, and the Doppler Effect, used for its wave principle. He used this knowledge to then state that galaxies are gradually moving away from us. Now, that isn't too unfathomable. Einstein himself knew his theory of gravity wouldn't work unless the universe was moving: expanding or contracting.

As the astrophysicists and telescope-owners looked further out into space, seeing images hundreds and thousands of light-years old, they also measured how fast those galaxies were moving, and how the galaxies interacted with each other. What they found was that the universe had a beginning. If the galaxies had been moving away from each other as far back as anyone could look at that area in time, then they all must have been together at one instance. Scientists call this the Big Bang Theory, which is a great deal of mathematical equations that prove the universe was at one point extremely small, hot, and dense. About 15 billion years ago, the universe blew up into elements and dust, and eventually stars, planets, and galaxies.

But if the universe has been expanding from the beginning, will it slow down? It can't. Gravity can't pull it back together. There's only 4% of the mass needed for that to happen. There was a theory that gravity would be able to pull the universe back together and it would end in a small, hot, dense ball. However much poetic justice exists in that theory, there is no chance of that happening.

The next question in all the intelligent beings' minds was how the universe would end. The expansion of the universe is getting faster and faster, accelerating from a point of about 10 billion years ago. And the people had no idea why. There's not enough mass for that to be a deciding factor on the movement of the galaxies, so maybe there is a massless factor.

That massless factor is called dark energy, because we can't see it (dark), and it would take a lot to move the whole cosmos (energy).

Now that the Big Crunch Theory is fairly refutable, there are two significant options as to how the universe will go on. One is that the galaxies will keep moving further and further away from each other until space is dark and lonely. It sounds rather like me, doesn't it? Even though most do not believe I can feel loneliness, I do. Perhaps I'll end like that. It would be boring, but that is to expected, because many think I deserve it.

But the other option. The other option scares me, honestly, because it too I deserve.

The dark energy makes galaxies rip from one another, pushes everything away from everything else, so it may rip apart every bit of matter, every atom, every molecule, every star and planet, until there is nothing left but energy. Invisible motion and lightless life; I will live through slowly being ripped apart.

You and I, John, are perhaps galaxies. The gravity and size make sense. However at the same time, we're different universes in the same universe. That doesn't make sense, does it? Humor me for the sake of metaphor.

We, in the same universe, are slowly being torn to pieces by dark energy, something neither of us can see, but the both of us know it's there. It's the secret no one knows, yet everyone knows about. Our universe is 73% dark energy, 23% cold dark matter (that's mostly mine), and only 4% mass. I said before that there wasn't enough mass, and gravity can't pull us back together again. The eternal attractive force has no power anymore.

However, I also said that we are separate universes. Why? You will die, but you will fade first, like the theory that space shall become dark and lonely. Soon, you'll die without anyone with you, but it's a kinder death than mine. I can't subject you to my own fate.

My fate is to die screaming, every atom of mine ripping apart and crumbling into dust until I blow away with the wind.

I learned the hard way that sentiment is a chemical defect found on the losing side, because the dark energy is currently and will forever use it against me. I can't be saved, John. I've made peace with that.

For now, all I can tell you is to forget about me. Dark energy is called dark for a reason: there is no light to illuminate it, and you're the brightest light of all if you ignore me and what will happen. I won't send this letter; you'll end up getting a much more brief and inaccurate one when you need to, mostly because I'm a coward that could not stomach you dying.

Moriarty will be my dark energy for now, but his ripple effect will last forever, while your dark energy is me, and will last only until I am gone. It will be very soon, don't worry.

You are my light, and I am your darkness. Let me go knowing that I will always be grateful for what you've done for me. I love you as a villain loves the hero, as the moon loves the sun, as Lucifer loved God. And, like Lucifer, I will not serve those that wish me to submit. I will fight for you until my universe tears like this paper, flung to the far reaches of the sky.

Forever yours,

William Sherlock Scott Holmes

The sky over St. Bart's is clear, only a few clouds to speak of. Sherlock can hear music playing from the corner of the roof; he thinks Moriarty has dismal taste, but that can't always be helped. "Staying alive. It's so boring, isn't it?"

Sherlock nods. He wonders sometimes if he and Moriarty are simply the same person from two different dimensions. Dimensional theory has always intrigued him. Perhaps it's based in fact. Perhaps he could ask Mycroft to look into it before he leaves. "Yes. It is. Unfortunately, there is no way for that to change."

"We can avoid the boredom in one way, Sher. You know what that is, don't you?"

He smiles. "I've known since the beginning. Billions of years, in fact. Everything dies, even stars, even white dwarf stars that have already died. It's time enough that I should as well. My heart is gone, you needn't worry about it."

Moriarty grins happily. "And whyever not? What reasons do you have to say that to me, the one that worries the most about you?"

Sherlock sighs. "I made sure to dispose of my heart before I came here so that it can live without me. I also hid the truth from it so it will never look for me."

The consulting criminal peeks over the rooftop. "I had to do my own disposal before we met as well. Don't tell anyone." Moriarty puts a finger to his lips.

"The dead can tell no tales, remember? Besides, I knew you had a heart."

"How, exactly?" Moriarty folds his arms across his chest, a snarky look twisting his lips.

"Every galaxy needs a black hole, my dear. We are the same."

He pauses, looking Sherlock over with those black eyes. "Nah. You're on the side of the angels, just like the others. You're ordinary."

"Sorry to disappoint. I may be on the side of the angels, but don't think for one second that I am one of them."

Moriarty walks toward him, saying, "You know that Luci himself was an angel, and he still is despite the fact he lives in Hell. Aren't you just like him?"

"Neither of us are like anyone. We are each other." Sherlock pulls out John's Sig Sauer, and beckons Moriarty closer. "We have to do this together."

Moriarty smiles, taking Sherlock's outstretched hand. "Heart to heart or head to head?"

Sherlock knows what his opponent is asking, and gently moves him so that their chests are touching. "I promised my heart I would die screaming, being ripped apart, and I am nothing but a man of my word. The head shot would be too quick and besides, dying by a shot to the temple is just boring. Humor me."

Moriarty nods. "I shall."

By the time they both got situated, Sherlock and Moriarty's thumbs are on the trigger behind Moriarty's back in case the shot doesn't go all the way through. However many things they have done to each other, they know instinctively that no one is leaving alone or alive. "Do you need to call anyone?" Sherlock asks, phone in his other hand. "People have this habit of wanting closure."

Moriarty doesn't answer for a moment. "He'll kill your heart if I give him a motive. Seb likes revenge more than I ever did. If you want to call John, it's alright. He won't be able to hurt anyone."

"Thank you." Sherlock dials the first number he has, putting the phone to his ear. He knows the letter won't reach John, so this is his last chance.

"Sherlock, where are you? I'm looking all around St. Bart's." It hurts to hear John's voice, but Sherlock doesn't want the last thing he hears of John to be 'you machine'.

"I'm on the roof, but you can't see me. And you can't move." He pauses, swallowing. "Is there anything you want to say? I don't want to go not knowing everything."

"Go? Who said anything about..." John trails off. "Sherlock, you can't. You can't, I won't let you."

"John," he murmurs. "It has to be this way. I promised you."

Moriarty suddenly motions for Sherlock, getting his full attention while John struggles for words. He whispers into Sherlock's ear a few words, and then wrenches the gun from his hand, pressing against his own chest and shooting.

Sherlock's mind temporarily goes blank, and he doesn't notice John talking to him, he doesn't notice the sky, he doesn't notice the wind whistling through his hair. "Keep your promise. Rip yourself apart." Moriarty lies dying on the rooftop, and Sherlock can see the bullet digging into the skin of his chest. But it's Sherlock that is in pain. Ripping, just like he said.

Sherlock knew Moriarty had given insurance that both of them would die on the roof. He'd given insurance through the three IOUs to Sherlock. He owed Sherlock three friends dead, and Sherlock had agreed to meet Moriarty for John. The deal the two of them had made with the single bullet was that if both died, everyone else lived. Sherlock will do what he has to, because of his promise, and because he is too much of a coward to watch John die.

"No, John, I'm still here. That gunshot wasn't me."

"Who was it then?! Tell me you're alright."

"I can't do that."

John growls a little under his breath. It's all Sherlock can do to stop himself from telling John where the letter is that says all the accurate information. So he cheats just slightly. "I can tell you something, though. Why."

"You shouldn't need to tell me anything! You should come down."

"I can't." He quietly pulls the gun out of Moriarty's hands. "I'm not going because Moriarty wants me to, although he does. I'm going because the dark energy pulling you apart slowly and killing me right now are the same thing. It's the poison I've contaminated the world with. I'm unnatural. I am the villain of this story, and you are the hero that I can't let go of."

"Don't you dare pull a fucking Skyfall on me, Sherlock. There is nothing wrong with you."

Sherlock shakes his head. "There I go again. Don't listen to me anymore." When John doesn't answer, Sherlock has to ask, "Have you stopped listening?" He hears nothing again, so he whispers, "I love you too much to drag you down with me. Lucifer was meant to be in Hell." He remembers not to brace himself. "Goodbye, John."

He doesn't bother listening either, throwing the Sig Sauer that John was always meant to keep off the roof, and his phone. When he thinks there is nothing left for him to stay for, he jumps, closing his eyes and relaxing into the rushing air.

John doesn't remember seeing Sherlock hit the ground. He doesn't remember catching his gun with both hands. He doesn't even notice his surroundings or the people flowing around him like water.

John remembers the sound of Sherlock's last words, and the detached sobbing of a man who has lost everything he holds dear. But John doesn't know whether the crying is his or someone else's. It feels so far away now, the few remnants of a memory that has faded too much but is still all too clear. "Sherlock!" he shouts into the early morning. "Sherlock, where are you? Sherlock!"

The sky doesn't answer, and yet, John thinks he can see dark streaks fly across it, and shreds of paper flutter through the air that has become too toxic to breathe. "Sherlock?"

Sometimes it's like his detective was never there in the first place. Torn from his grasp, and softly lifted from everyone else's. Why is it meant to be painful for him alone?

When Sherlock wakes up (and he didn't mean to wake up), unwanted tears begin to run down his face. "Mycroft!" he screams. "You weren't supposed to save me! How could you do that to me?! How could you save me?!" The orderlies strap him down and stab a sedative needle into his neck, but he can't stop screaming. Somewhere, he knows he didn't want to survive. The mighty fall, it's how the story goes.

Moving hurts, so he moves as much as he can anyway before the sedative kicks in. Sherlock has to make sure that this isn't real life, that he made it to Hell. As the hours pass, and his sedatives and straps linger, he knows that this is real. "Why did you do it?" he asks to the wall. "I can hurt people still. I'm still a machine, I'm still a monster that watches murder for sport, I'm still a man meant to corrupt angels."

"No, Sherlock," Mycroft answers after an indeterminate amount of time. "You're a man that loved too much, didn't take enough depression medication, and believed in the power of martyrs."

It's horrible that he has been reduced to this, but he recognizes the truth in his brother's words. "What am I supposed to do now?"

Mycroft smiles, but it's not like John's smiles, when his entire body is warm. His body feels cold and immovable with one of Mycroft's smiles. "Tear Hell into pieces. You said you wanted to go there, and now, here's your chance. John won't have faded too much by the time you get back."

"So what's the point of all this?"

"The point is how easy it is for a single force to leak through barriers and run rampant with its power. In this case, your sense of protection and martyr complex. However much you want to believe Moriarty or yourself is the problem, it always comes back to sentiment. You want to save the world for John, Sherlock. You want to be the hero no matter how many times you have denied it."

Sherlock stares at him intently. "If I be the hero, if I run through Hell, will I be able to save John?"

Mycroft nods. "You will. But not even gravity can pull you back together. There's not enough mass."

Of course his brother would have read his private letter, but Sherlock stands his ground. "Every galaxy has its own peculiar velocity. We'll come back together another way."

Wow. I had no idea that would end up that sad. Sorry about that. Hopefully y'all will read + review anyway!