More Information about dark matter

Dark matter is a pillar of modern cosmology, that is, the bringing together of astronomy and physics to understand the physical universe as a unified whole. Extensive astrophysical evidence indicates that the matter that we are familiar with, the stars, the planets – everything – including ourselves, is but a small fraction of the total ‘material’ of the universe. These observations are, by definition, observations of the material we can see. Construction of a physical model that is consistent with all these observations, at the same time as being consistent with the known laws of gravity, is most easily achieved (arguably, only achieved) through the inclusion of materials known as dark matter and dark energy. Theoretical particle physics provides a compelling case for there being fundamental particles in the universe around us, likely created together with the normal matter at the time of big bang. These have exactly the properties and abundance needed to explain the observations. If such particles do indeed populate the universe around us, an extremely sensitive detector, shielded from background radiation, may be able to directly detect them, thus providing confirmation of the particle dark matter hypothesis, allowing characterisation of the new particle’s properties, and furthering our understanding of the true nature of the universe as a whole.

What is Dark Matter?

Since Newton and Einstein, physicists have developed mathematical laws that describe how we expect things to move due to gravity. Unfortunately, when astronomers look at the motion and distributions of stars and galaxies, these laws just don’t work. It turns out though that if, in addition to the normal material we can see in the universe, there is a large amount of invisible material, then the laws of Newton and Einstein work just fine, for galaxies, for clusters of galaxies and for cosmology as a whole. That this solution works over such a range of scales makes it particularly compelling. This extra material is dark matter, and there’s about six times as much of it as the normal matter that we can see. What the actual dark matter is, we don’t know. We do know it can’t just be normal matter that for some reason we are not seeing - it has to be something completely new to physics. Many theories that provide a framework for understanding the particles that make the normal matter in the universe also predict new particles with exactly the properties needed for dark matter.

Why should we care about Dark Matter?

Whether there is dark matter, and what it might be, are big problems in physics and astronomy. But our overall understanding of the universe, and what it is made of, has many more problems too, for example, what is the dark energy that appears to be driving the universe as a whole to expand ever faster? Why is the universe filled with matter and not antimatter? Why do the particles we do know about, like electrons and quarks, have the properties they do?. Understanding dark matter may be the first step in a genuinely deeper understanding of Nature, with consequences far beyond physics and astronomy.

Is Dark Matter surrounding us now?

Yes! The motion of the stars in our galaxy, the Milky Way, only makes sense if there is a large amount of dark matter in addition to the matter we can see in stars, planets, dust, etc. We can’t see (or feel) the dark matter because it interacts incredibly rarely with normal matter - it just zips right through us. We think there’s something like a few dark matter particles per cubic metre around us, moving around at speeds of tens of thousands of kilometres per second!

I've heard dark matter might be made of WIMPs. What are WIMPs?

WIMP stands for Weakly Interacting Massive Particle. Astronomical evidence has allowed us to determine a fairly specific list of properties for dark matter, if it exists. A popular idea for what dark matter might be is that it is made of fundamen tal particles that would be relatively heavy compared to the particles that we know of such as electrons and protons, hence Massive Particle. They then interact so weakly that existing detectors don’t have the sensitivity to see them. There are other possibilities for what the dark matter might be too, with exotic names such as axions or dark photons. The Edinburgh dark matter group has a particularly strong role in searches for these even more exotic dark matter candidates.