[nuclear_eclipse]

If you can answer the why, then I think physicists around the world would really like to know. We "know" the universe is expanding because we can observe the effects (eg, red shift of further away stars/galaxies), but the "why" has been the big, fundamental question for as long as it was predicted and/or observed. It's almost certainly related to some property of physics that we don't yet understand, but ultimately, we may never know any reason beyond "that's just how it is".

[chanakya]

Thanks. So it's not related to Einstein's General Theory? In discussions around the cosmological constant, static vs. expanding universe and energy of space, it feels like the expansion is a consequence of the General Theory, and that all we need is to fill in some constants, like the amount of matter in the universe, and the actual energy of space.

Is the theory missing in some more fundamental way, like the lack of unification of gravity and quantum mechanics?