Look around and you might believe that the universe is made entirely of matter – substances that have mass and take up space. We see it almost everywhere.
But whilst the universe does indeed contain matter, this is only a tiny amount of its composition (the parts it’s made of). The matter we can see is called ‘visible’ or ‘normal’ matter. The universe is made up of only 5% of it.
So, what else is there? The rest of the universe is actually made of stuff we can’t see – dark matter (27%) and dark energy (68%).
- Dark Matter
The idea of dark matter is hard - there is stuff in space which we know is there but cannot see! We see the effect of dark matter on the objects around it. We know it must have mass, and therefore gravity.
One example is by looking at how spiral galaxies spin. It has been found that the mass needed to hold galaxies together is far more than the mass that we can see. There must be a large amount of unseen mass, or dark matter. In some cases, around 90% of a galaxy must be made of this dark matter.
Scientists still do not know what dark matter really is. Some think it's made of long-dead stars, unseen planets and black holes. But these are made of particles we can usually detect. Others suggest that there are new, strange particles, not yet seen. It could even be that the laws of physics which we rely on, break down at long distance scales.
There is still a lot of research in this area to see why the measurements of mass due to light and due to gravity do not add up.
- Dark Energy
We know that the Universe is expanding. When we look at other galaxies, they are moving away from us. And the further away the galaxy, the faster it is accelerating away. The expansion is speeding up.
Astronomers don’t know yet why this happening, but they have a theory – a force called dark energy that is pushing everything in the Universe apart.
Energy and mass can be thought of different forms of the same thing (this is what Einstein’s equation E=mc2 says). Mass can become energy, and energy can become mass. This is why we include dark energy in the composition of the universe.
There’s still a lot of mystery around dark energy. Astronomers are trying to understand what it is and how it works.