Dark matter. The stuff that (allegedly) fills the universe, yet it cannot be seen, felt or detected. So what's out there?
I understand it's hypothetical, but I tend to hear it talked about as theory (dictionary definition- a theory is a group of tested propositions; often confused with hypothesis). Personally I think dark matter is something basically made-up to make the equations work. It supposedly fills the universe and is responsible for it's expansion, but where is it?
I realized I should probably do some more research first, so I turned to Wikipedia. "Dark matter... accounts for most of the matter in the universe." "... inferred..." "... hypothesized to account for discrepancies in calculations." Here they say it's hypothetical, but on The Universe and How the Universe Works (Science channel-I'm quadripalegic so I don't work) they talk about it like it's a sure thing.
So what do I think is responsible for the universe's expansion? Simple. Conservation of momentum; not the stuff in the universe, but the fabric of space-time itself.