Thread: How do we know everyone dreams?

Whenever someone around here says "I don't dream. I don't remember anything when I wake up," there's always someone else saying "Yes, you do. Everyone dreams. You dream, even if you can't remember it." How do we actually know that everyone dreams? What's the evidence? Seems to me like scientists observed brain activity of people while they sleep and assumed that, just because everyone has REM cycles, everyone dreams. REM cycles hardly prove the existence of dreams, though. We know that it's possible to go into REM without dreaming.

So, I ask again: What's the evidence?