10 Biggest Unsolved Mysteries In Science

2. What Is Dark Matter?

Dark matter accounts for about 85% of the matter in the universe ... and that's pretty much all we know about it.

It neither emits nor absorbs light anywhere on the electromagnetic spectrum, it is completely invisible to the naked eye and specialised telescopes, you can't touch it taste it or smell it, yet we know it's there.

Whilst we can't observe dark matter directly, we can see its effects throughout the universe. We can see the gravitational forces that it exerts on galaxies, causing them to cluster, and even the gravitational lensing that occurs as it bends spacetime around it. Scientists have even been able to map its distribution throughout the universe.

All without a bally clue what the stuff is. As far as we're concerned, the only property it has is mass.

When combined with the equally unhelpfully enigmatic dark energy, the two account for somewhere in the region of 95.1% of the mass-energy content of the universe. Everything else we see and experience is made up of the other 5%.