Dark Matter

Our universe is a weird and wonderful thing. It popped into existence 13.8 billion years ago, for a reason which is still beyond us, and continues to hide other secrets which physicists struggle to unravel. Amongst these mysteries lies a question which is seemingly quite simple: what is our universe made of? The answer might surprise you: largely not the same stuff that makes up me, you and anything that we can see. 

According to analysis of microwave background data collected by the Planck space observatory, the composition of our universe looks something like this (see chart below). 

Source: Department of Physics, University of Oxford

You may be surprised to learn that 95.1% of everything in existence is something you probably haven’t heard of: Dark Matter and Dark Energy. 

When physicists say something is “dark”, what they mean is that it can’t be observed in the way that normal matter can. In other words, it doesn’t emit energy or light, and the only way scientists know it exists is because of its gravitational effects. To be more precise, we can suppose dark matter exists because of the speed at which stars move in galaxies. 

Stars’ movements and their speeds can, in fact, be accurately predicted since they are the result of gravitational forces exerted by other forms of matter in a galaxy. Hence, when astronomers sum all the matter they see in stars, gas and dust clouds, they can use this figure to calculate the speed at which stars should be moving.

In reality, stars move much faster than they should be. In other words, there is insufficient observable matter to produce the gravitational effects which would make stars move so quickly. The only logical explanation can be that there is other (“dark”) matter which we cannot see that produces this extra gravity. 

What exactly is this dark matter then? Since we know so little about it, it is almost easier to list all the things we know it isn’t. For example, since we can’t see it, we know it doesn’t interact with the electromagnetic force. We also know that dark matter is not antimatter since we don’t observe the unique emissions of gamma rays which are produced when matter and antimatter interact. 

Until more is known about this mysterious entity which makes up 26.8% of our universe, I’m afraid we might have to be satisfied with our understanding of regular old matter.