Penny: What got you excited about dark matter in the first place?
Sheldon: Well, I left string theory, which I'd been working on for a long time, and everyone was talking about how cool dark matter was, and I thought, "€œWell, sure, I'll give that a whirl".
Penny: So it's your rebound science?
Sheldon: What's that?
Penny: Well, not the science you spend the rest of your life with, but the one you use to make yourself feel pretty again.