Dark energy, as explained by Steven Weinberg

One of the things I've always wanted to learn more about is astrophysics. I don't talk about the universe much here. So today I decided to go to a lecture at UT Dallas featuring Steven Weinberg, who won the 1979 Nobel in physics for his work on explaining the unification of electromagnetism and weak force. At the end, an audience member asked him how the universe could be expanding when gravity pulls galaxies together. Here's how he responded, and note that I'm only paraphrasing.

Twenty years ago, he would have said that gravity isn't strong enough to bind objects together. As with a rocket that propels at several miles per second to escape Earth's gravity, two galaxies are speeding up at such a fast pace that it's impossible for gravity to keep a hold on them.

Now, he said, theoretical physicists like himself explain this phenomenon by citing a yet poorly understood concept known as dark energy. This form of energy actually goes against gravity in separating two objects, thus making the universe bigger.

Reading the Wikipedia articles on dark matter and dark energy won't help me; I don't know much about astrophysics to begin with, although I do at least know the general idea of how the universe was formed and expanded thereafter, making way for galaxies to appear. I need to buy his book, The First Three Minutes. That'll help me.