"Already 99.9 (and about 58 more 9s) percent of the universe — it is expanding lickety-split — is beyond Earth's atmosphere.
Into what is it expanding? Hard to say. We can say there is lots of stuff in space: Hold up a penny at arm's length and you block three galaxies from your field of vision — billions of stars and other things — 350 million light-years away, which is right next door in our wee corner of the universe.
But there is much more space than there is stuff in space: If there were only three bees in America, the air would be more crowded with bees than space is with stars. But there is much stuff besides stars whizzing around, and 65 million years ago — the day before yesterday on the calendar of the 14-billion-year-old universe — big bits of stuff entered Earth's atmosphere traveling faster than a high-caliber-rifle bullet."