Hi Jack- Since your questions are so closely related, I've lumped them all together here.
"what does the universe consist of?"
The universe is filled with various forms of energy. About 4% is
the rest energy of the ordinary sort of matter we're familiar with.
About 25% is some other type of matter, called dark matter, which has
ordinary gravity but otherwise doesn't interact much with ordinary
matter. We don't yet know much about the dark matter. The rest is
something called 'dark energy' about which we know very little indeed.
It has the unusual property that its density stays the same as the
universe expands. Ordinary stuff gets less dense as space expands.
"how is the matter in the universe arranged?"
On a very large scale, it's nearly uniform. On smaller scales it's
lumpy. For example, there are galaxies and clusters of galaxies.
" how big is the universe? How many stars are there estimated to be?"
We don't know if the universe is finite or infinite. The part we
can see extends out about 14 billion light years from us. We're pretty
sure, based on our understanding of how it got to be so nearly uniform,
that the mathematical space it is in extends out at least 100 times
farther. Unless something unexpected happens to the laws of physics,
that extra part will never be visible from here.
In the part we can see, I believe that there are something like
10^21 stars. Considering the extension that we are pretty sure must
exist, there should be at least 10^27 stars. However, so far as we can
tell it may just go on forever, with an infinite number of stars.
(republished on 07/19/06)