So far we’ve mainly been talking about light – but is this all there is in our Universe?

It may come as a surprise to you that everything we can see, all the stars and galaxies, are just a tiny fraction of everything out there. There are two “dark” components to our Universe.

 

Dark matter is a substance required to explain the masses of galaxies and clusters. These produce effects like gravitational lensing, from which scientists can determine their mass. Visible matter is insufficient to explain what’s observe, so theoretical physicists have come up with various explanations for it: invisible particles we haven’t yet discovered, a problem with Einstein’s theory of General Relativity, and more unusual hypothesis include string theory, according to which all particles are strings that vibrate, much like a guitar string.

A gravitational lens observed by the Hubble Space Telescope

This is not to be confused with dark energy, which is the force that causes the Universe to expand at an increasingly fast rate.