Cosmologist Risa Wechsler of the Kavli Institute for Particle Astrophysics and Cosmology explains.
Have a burning question about particle physics? Let us know via email or Twitter (using the hashtag #AskSymmetry). We might answer you in a future video!
You can watch a playlist of the #AskSymmetry videos here. You can see Risa Wechsler's answers to readers' questions about dark matter and dark energy on Twitter here.