The foundation of much of the galaxy's modern technology, element zero, commonly known as eezo, is less an actual element than a state certain atoms are in. While an element zero atom has mass, it completely lacks a gravitational field of any form. This and eezo's effects are believed to be caused by dark energy trapped within the atom during a supernova being released and altered through the positive or negative electric current passed through the eezo. This produces the effect of either increasing or decreasing an object's mass depending on whether it is a positive or negative electric current passing through the eezo.
The eezo does not actually create energy, as some believe, but shifts the effects of mass into energy and vice versa. A bullet shot from a gun will have the same force of impact whether it hits an object with an altered mass or not, it will just be going a different velocity and mass differently. If the mass effect field around an object disperses, then the mass and velocity will correct, rapidly shifting both to their original ratio. This is what allows vessels to accelerate and decelerate relatively quickly, simply changing their mass alters their velocity, giving ships control of their speed without needing counter burns, though they are still needed to come to a complete stop.
With this effect, a vessel can move at speeds far greater than their engines would normally allow for, greatly reducing travel times and cost.
Another effect eezo can have, with precise use, is the simulation of gravity through hyper thin layers of matter up massed to the point they equal the gravity of a planet. This effect decays rapidly with distance, lowering the effective gravity and creating the odd effect of perceptibly lower gravity at the top of a person and their bottom, something that can be disorienting to first time space travelers. Many individuals who are expecting to spend a lot of time under such gravity such as ship crews and members of the Alliance take augmentations to better acclimate to the effect (this is also the reason most space stations, that can, go for rotational gravity instead of eezo gravity alongside the cost concerns). The creation of a mono-gravitic field is a complex endeavor, but most species discover it through reverse engineering prothean technology. Mono gravitic fields are vital for the operation of personal kinetic barriers, with them being an impossibility without them, while also making artificial gravity and other technologies more challenging and less effective. Mankind managed to reverse engineer the technology in 2151, only three years after discovering the Prothean Ruins on Mars. However, mankind has still not learned how to create personal kinetic barriers as of first contact.
Any element can theoretically be eezo, though with some caveats: radioactive eezo reacts poorly to being used, often causing effects similar to if they were used in a fission reactor. While this has been used in some truly interesting early Asari nuclear technology (and also leading to the Asari gaining fission tech far earlier in their development than other species, roughly equivalent to humanity's mid-to-late 19th century), it makes radioactive eezo unappealing for use in eezo cores. Additionally, gaseous eezo has proven extremely difficult to work with, making its use relatively limited. Additionally, eezo eventually decays into its normal element after a long period of use, running out of dark energy within it. The amount of time depends on the amount of use, but it usually reaches into the centuries or even millenia (though the latter is only found with high amounts of disuse). The Mass Relays and the Citadel don't appear to have the same limitation, somehow reenergizing its eezo. This ability is not seen within any other prothean technology, with only the newest prothean technology showing slowed eezo degradation, though not stopped like found in the Relays and Citadel.
Eezo can be found in concentrations spread out across all samples of matter, ranging from near nothing to almost half eezo. The latter is so rare that any large source, such as Omega, becomes a location of national importance to even the Council Races, with entire fleets sometimes stationed for their defense. Most sources mined range from 1 to 35 percent eezo purity, with those over 35 being rare and those under 1 being uneconomical to purify, though some eezo poor areas will see mining of lower purity eezo.
Eezo can be found in high amounts, though in varying purity, around in Neutron Star and Pulsars. These systems hold an, on average, far higher overall concentration of eezo than most other systems, with average eezo concentrations being five to a hundred times that of the galactic average for star systems. Mining in these systems is very expensive and dangerous, with most such systems being relatively distant from Mass Relays and the star's themselves producing extreme amounts of radiation. This leaves these sources only available to the largest corporations, leaving most eezo production to sources spread across the galaxy.
Clusters range wildly in their eezo concentration, for instance; the Gemini Sigma is extremely eezo poor, with few known sources worth mining, while the Athena Nebula holds the highest concentration in the galaxy, a great source of Asari influence. It also varies between systems within a Cluster, usually caused by stars traveling into a Cluster with a greatly different concentration than its own. For instance; the Sol System holds relatively little eezo, with only a few known sources worth mining, while the Altair System is rich in eezo, among the top 5 percent in concentration, with Sol in the lowest quarter.
All sources of eezo must be purified to some degree before being used in any form, with concentrations lower than 5 percent not even working when given an electric current. The purification process uses a mixture of electric currents and centrifugal force to extract the eezo from the normal elements. This is a relatively costly process at scale, with entire space stations constructed to allow for industrial eezo purification. Smaller scale purification can be done with relatively simple equipment, commonly used on survey ships and drones to determine eezo purity, though many vessels simply pass an electric current through an asteroid before testing it, a simple and cost effective, if wastfull due to common false negatives, method.
High concentrations of eezo are toxic to most races, with them causing mutagenic effects, especially in unborn children.