Having mass is the one thing that’s really certain about dark matter. Dark matter's existence was discovered by measurements that meant that there had to be some extra, invisible, material, which was contributing mass to the galaxies we were looking at. What kind of material it is exactly remains a bit mysterious, but we’re pretty sure it should be some kind of fundamental particle, and many ongoing experiments are being run to try and narrow down what that particle could look like. It seems that dark matter only interacts with the rest of our Universe’s atoms (like the hydrogen of a gas cloud, or the iron in your blood) through gravity.
We also know that dark matter does cluster together, because the gathering of dark matter into massive, unseen spheres within our cosmos are the birthplaces of galaxies. Every galaxy we’ve looked at with sufficient precision has told us that there is more dark matter than there is luminous matter. In order to directly measure dark matter, the easiest methods involve measuring the rotation of the galaxy, which we can only do for nearby galaxies, or by measuring a gravitational lens, which we can only do for very massive, geometrically lucky galaxies. But where these measurements are possible, dark matter surrounding our Universe’s galaxies seems to be omnipresent. It’s to the point where the presence of dark matter is one way of defining what a galaxy is -- if the collection of stars is too small to have its own surrounding nest of dark matter, it can’t be a formal galaxy. As we get a little more cunning about how to look for faint galaxies, we are starting to find darker and darker galaxies, which are comprised of more and more dark matter and less and less luminous matter.
But to make a dark star and not just a dark galaxy is a bit harder, because stars have to collapse down in a very small region of space. If your star is made of normal matter, part of this collapsing process occurs because the gas particles cool down, which allows them to take up less space. But dark matter particles don’t seem to interact with each other or with normal matter except through the force of gravity -- these dark matter particles can’t shed heat and become more densely concentrated.
From a purely theoretical perspective, it is possible to have a star with a significant fraction of dark matter inside it. However, we still don’t know exactly what dark matter is. The different dark matter options which remain open to us can change its behavior in extreme situations – a dark star would certainly be one of these extremes. One of the options is that dark matter could serve as its own antiparticle, meaning that if two dark matter particles collide, they have a chance of converting themselves into high energy forms of light.
It’s by no means clear that this is how dark matter functions, and most simulations of the dark universe don’t allow for this dark matter-to-light conversion. But if it is the way that dark matter functions, then a high concentration of dark matter could produce rather a lot of high energy light, which in turn could heat any nearby gas and dust. This gives rise to bizarre approximation of a star, powered by dark matter destroying itself, instead of the fusion process which heats our own star.
There’s no observational evidence for these ‘dark stars,’ but it’s always interesting to know what kinds of objects might be hiding out in our Universe, given how much physics we already understand, waiting to be known.
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