Sans had always known about the many different realities of the Underground. He had learned by accident once upon a time many timelines ago, back when he actually cared about getting their world out of the dominating grasp of determination and all the things that came with it. Back then, he had been one of the two assistants of the Royal Scientist, W.D. Gaster, his father; the other assistant was Alphys.
It was just a regular day. Gaster was, as usual, working hard on developing counterfeit human souls to try and "trick the lock" of the barrier trapping monsters Underground, roughly a year after the War of Races between the monsters and humans. He, Sans, and Alphys were all just about to make a breakthrough, too. They finally learned that adding a touch of monster SOUL pigments with the humans' was the key to actually keeping them in tact. After all, humans and monsters were always meant to live together in harmony as they relied on each other's powers to thrive: monsters relied on the humans' determination and other traits such as justice and kindness, as humans relied on monster magic.
Unfortunately, they never got the chance to actually create the seven fake human SOULs. The Core had overheated despite the efforts of a few monsters in Snowdin sending blocks of ice via the river to the area to keep it cooled, and the whole facility spiraled out of control. Gaster had held Sans and Alphys back as he tried to steer it back into control, but instead a beam had collapsed into the balcony. The Royal Scientist had lost his balance, and before his assistants could even move he fell into the Core, never to be seen again.
After a blinding white light and a devastating explosion, everything went black.
When Sans came to, he found himself laying on the sofa in his house. Apparently, he lived in Snowdin Town with his younger brother Papyrus rather than in Hotland with his father. But that was the weirdest part. The weirdest part is that no one even remembered Gaster. Sans had tried to approach Papyrus about the destruction of their father, but his brother didn't seem to remember him at all. No one remembered him. No one even remembered Sans being his assistant. According to monsters, Alphys had always been the Royal Scientist, and she didn't have any known assistants. When Sans tried to get in contact with her, she didn't seem to recognize Gaster, either. Heck, she barely even remembered Sans himself, and they were best friends during the time before Gaster fell!
Sans was completely at a loss. He had no idea what was going on. How could someone falling into the Core screw everything up? Well, Sans was determined to find out exactly what the hell happened to their world. There was an unused room in the back of the brothers' house, which was the perfect place for a mini lab. Sans bought a key and padlock to attach to the door and keep anyone - especially Papyrus - from sneaking in whenever he was gone. He gathered many materials from Hotland specifically to build a machine, making sure to use his and Gaster's old secret language to ensure no one could make out the words. This machine was to be used to go back in time and prevent Gaster from falling into the Core. He drew out many blueprints for the time machine, and once he was satisfied we went to work immediately. However, that wasn't before he drew a little sketch of himself, Papyrus, and Gaster. He wrote, "Don't forget" on the bottom right corner, and once he finished he hid the drawing in a drawer under his blueprints and other notes he had to rewrite from the past timeline.
It took Sans way too many timelines to finally complete the time machine; he learned there was a mysterious force that could turn back time at any given moment, but the source or controller of this power was unknown. He made sure to write down as many notes on the phenomenon as possible, and he made sure never to take any of these notes out of his workshop as it seemed to be unaffected by the anomaly. He had decided to call these strange occurrences RESETs, as they were similar to a game being reset by a player. Each time these RESETs occurred, Sans noticed that everyone would act the exact same way they had the timeline before, almost as if they were some kind of NPCs of a video game.
Once the time machine was finally completed, he decided to test it. Crossing his fingers and hoping his invention would work, he switched on the power and waited as the energy built up until it had enough juice to summon a colorful vortex-like wall. He was successful enough to create a portal! So far so good.
But what Sans saw on the other side completely baffled him. He was staring back at something. No, at someone. The being on the other side was wearing what looked like some kind of very light armor; it was a little hard to tell with all the swirling colors of the portal. The being was wearing a shirt with gloves and boots, as well as shoulder paddings and a bandana around his neck. This being was also a skeleton.
And this skeleton looked just like Sans.
Sans leaped back in alarm, sockets widened. He had absolutely no explanation for what the heck was going on. He was so distracted by the strange occurrence that he didn't even notice the smoke rising from the back of the machine. It was beginning to malfunction. It was using far too much power from the Core, and it caused the machine to overload.
It exploded and broke down, which scared Sans nearly half to dust. Once he gripped his bearings he began to silently curse himself for becoming too distracted and not turning off the machine before anything like this could have happened. Now he had to rebuild the machine all over again, or rather patch it back up.
But what if it didn't work the next time? It didn't work this time; instead, he just saw some... copy of himself. He had spent too much time on this damn mess of a machine, and frankly he wasn't too keen on trying again only to expect another failure. Besides, what were the chances it would break down again when he went back and couldn't return, destined to hide away for the rest of his natural life just to avoid meeting his past self and disturbing the space-time continuum?
Sighing in defeat, Sans slumped out of the workshop and returned a few minutes later with his pink bed sheet. He draped it over the machine to cover it up. His project was a failure, and he wasn't too keen on looking at it again so he could be reminded of how much of a failure he was himself. He had given up trying to fix the world.
He had given up on everything.
Sans locked the door to the workshop once he stepped out and went back inside the house through the front door. Papyrus was out recalibrating his puzzles in Snowdin Forest in hopes to catch a human in one of them, so he was all alone. He trudged up to his room and stepped inside once unlocking it, and he approached the drawer next to his bed. He pulled the key to his workshop out of his pocket, looked at it for a while, and sighed as he dropped it carelessly in the drawer and closed it.
With all hopes of fixing the world lost, Sans flopped into his messy bed and fell asleep.