Back in high school, I asked my teacher gave us a quick explanation of relativity. Specifically, he told us what $E=mc^2$ meant. He explained that, at least as far as we needed to be concerned, matter is simply condensed energy. This, to me, was amazing.
Obviously, it's not exactly that simple. Condensation isn't something that just happens with energy. However, it does mean that the forces which act on matter also act on energy.
Now, I am aware that gravity is a relatively weak force. Years after taking physics in both high school and college (though never at any real "scholarly level", I'm afraid), I remember asking my teacher something that made him smile and admit "I cannot answer that, and that is why I am a high school physics teacher and not an applied physicist. Questions like that are going to get you very far in this field."
That question was this: If gravity acts on all matter and all energy in the universe, wouldn't the universe eventually condense into pockets of energy and matter, after entropy causes the "heat death" of the universe?
Edit: better wording
Why wouldn't the same coalesce nice of matter and energy happen after the heat death of the universe?
If not, why?
Please note that I don't necessarily believe that the high-entropic state of the universe in billions and billions of years will coalesce into matter. I just want to know why entropy would continue.