"...it's true because almost everything in the universe is made up of matter."

Well, the last estimate I read was that only about 4% of the universe (excluding 3D space itself) is actually matter, so most of the universe is believed by scientists to be stuff that isn't matter, like energy, 'dark matter', 'dark energy', etc...

Light isn't matter, and everything you see is light. You don't see matter, you see the light that it radiates, or reflects.

You've never seen matter.

Weird, huh?