[neutralino]

HBD,

Firstly, I should point out that you mean astrophysicists, or astronomers, not cosmologists. Astronomers/astrophysicsts study the "stuff" in the universe, whereas cosmologists study the actual universe and its evolution.

Secondly, Einstein doesn't really say that mass curves spacetime; this is just the popularised way of thinking of things. The correct statement would be that anything with energy, or momentum, curves spacetime.

If you've ever studied GR, then you'll recall that Einsteins equations have two sides; a curvature part and a "stuff in the universe" part. The curvature, expressed as the Einstein tensor is proportional to the "stuff", which is expressed as the energy momentum tensor. This simple looking equation is really the crux of general relativity.