Delete the colon after know (and replaced it with a question mark). An ellipsis before the question mark would be okay.
Do not hyphenate 225 million. (You can write that out as two hundred twenty-five million if you like.)
Change rotates to revolves. (Yes, these are very different terms; the first means spinning in place, basically, and the other means going around in a circle. Rotation is what causes Earth’s day and night, but revolution is what gives us years.)
If you want to keep that last line, capitalize Now and add end punctuation.
Did you know…?
The solar system revolves around the galaxy once every 225 million years. The last time we were in our current position, dinosaurs were just beginning to walk the Earth!
…Now you do.
(Yeah, I know this is highly inaccurate, scientifically, because of many factors. Gotta start somewhere, this is almost certainly not aimed at an audience who knows much science anyway, and kudos to the writer for daring to mention dinosaurs, because these days, even that could provoke the wrath of excessively anti-intellectual types.)