I've thought about this many times. Regardless of your scientific/religious beliefs, I don't think it can really be explained. I think the major obstacle is that we are not wired to understand infinity backwards. I mean, it's pretty easy to think that there will always be "something" in space from now to infinity, but it's difficult to think that there was something in space from now to negative infinity. Everything in our life has some starting point, everything. I don't think we can justify something as to not having some sort of beginning. It defies logic, thus can't be explained.