the point about elerium is basically this: according to 1990's lore, there are theoretical "islands of stability" outside of the known table of elements, and element 115 (that is, an atom with 115 protons in its nucleus) was a reasonable candidate. it is a super-heavy element, but within theoretical possibility.
element-115 has been part of UFO/SciFi lore before XCom. the theories differ as to where it's coming from: in theory, any element can be created artificially, but also occur naturally through fusion processes. so, element 115 might occur naturally within stars, or after a supernova, or perhaps (if it really is stable) somewhere in asteroids or on planets that have formed from matter that originated in a supernova, or perhaps after a neutron star collision, or whatever...
XCom only told us that element 115 "does not occur naturally within our solar system". I think the later games mentioned something about stockpiling of Elerium after the first UFO wars, and a global energy crisis that resulted from the dwindling supplies (because after the UFO wars, a lot of the technological breakthroughs were based on elerium technology).
so we can safely assume that the aliens need to ship it into our solar system from the outside (or perhaps they had large stores of it on mars, which were destroyed after the brain was killed).
