Dark Matter

Evidence for Dark Matter

Here is a rotating spiral galaxy. We can measure how fast it is rotating by measuring the red and blue shift of the light from either side.
(Red shift = moving away, blue shift = moving towards).

galaxy pic - NASA

If we know how big the galaxy is as well we can, using Newton's law of gravitation, calculate its mass. The problem is that using this calculation it works out that there is a lot more mass in the galaxy that we can actually see. Could it be that there is a lot of invisible matter in the galaxy, indeed in the Universe?

What is Dark Matter?

We can see stars because they give off light. What about all the stuff out there which does not give of light, or any other type of detectable radiation? This is dark matter. Some scientists think that as much as 95% of the Universe may be made of dark matter. Certainly our galaxy contains a lot. We cannot see much of our galaxy because it is obscured by dark matter. We have learnt more about the cosmos in the last 50 years than we did in all recorded history before but, despite this, the majority of what is out there is a BIG mystery.

Some of it could be the same material that the planets are made of and a lot could be dead stars, brown dwarfs and the like. It is thought that at the centre of our galaxy, and most other galaxies, is a super giant black hole.

screenshot from the 1922 film Nosferatu	pic NASA
There is an awful lot of stuff out there we just can't see. We call this material Dark Matter.	We know of dark matter from its gravitational effects on the matter that we can see. Here it acting like a lens, bending the light from galaxies behind it.

How much dark matter is actually out there?

This is a very important question. If there is enough dark matter then, because of its gravity, the Universe may stop expanding. It may even start to start shrinking and eventually collapse. Don't panic though, astronomers now believe that the Universe will keep expanding.

More About Dark Matter (not needed for GCSE)

There are 2 distinct groups of dark matter, WIMPs and MACHOs

Weakly Interacting Massive Particles (WIMPs) are particles that have mass but don't interact strongly with other material, e.g. neutrinos, billions of which pass through our body every second without us noticing them. They are mostly detectable by their gravitational influence.

Massive Compact Halo Objects (MACHOs) are objects that populate the halo around the galaxy. These are planets and black holes which are basically invisible. There doesn't, however, appear to be enough MACHO material to account for the "missing mass" mentioned above.