The formation of large scale structures in the universe is probabably the problem requiring solution in cosmology. I was fascinated by the purely theoretical question of developing a formalism to study statistical mechanics of gravitating systems - both (i) in the context of isolated Newtonian case as well as (ii) in an expanding universe.
You will find most of my results, views etc regarding (i) in the Physics Reports review I wrote in 1989 [ref 77 in my list of publications] as well as in some of the papers, especially ref. 71.
I then started working on the statistical mechanics of gravitating systems in an expanding background and I believe I actually understand the physics at the level of power spectrum [see ref.101, 117,118] (This is important since some people in this field use computers the way drunkard uses a lamppost - for support, not illumination.) The key ideas are summarised in two pedagogical reviews:
More recently, I am trying to understand the issue of cosmological constant since observations seem to force us to take its existence seriously. My views are available in the recent reviews i wrote ref. 153,191,198.