Hello, I'm a 21-year-old male. I do not smoke and I drink only occasionally. For the past year I've had a small white bump on the roof of my mouth. Not long after I first noticed it, picked the white head off; it soon came back. It's not easy to pop like a pimple. The head feels sort of rubbery and a bit hard. I haven't messed with it since. I did mention it to the dentist around a year ago, but he didn't seem to think much of it. Here is a picture: i.imgur.com/liq1XbF.png

It has me pretty worried that it might be something serious. Any idea what it might be?