Is Outer Space Completely Empty?

The space between planets, or between stars, only appears to be empty.

Actually, it’s filled with tiny particles of gas or dust too small to be seen. In fact, about ten percent of all the matter in our galaxy is gas and dust particles floating through space.

But these particles are scattered very thinly through space. A thimbleful of our air contains trillions upon trillions of atoms, but a thimbleful of outer space may hold just ten atoms!

If a sphere the size of earth were filled with outer space, it would weigh only eight pounds!