Every now and again, you'll read a science story claiming the "best picture of dark matter" ever. If it's so dark, how can you see it? In this week's "Ask a Physicist", we'll find out.

I've talked a lot about dark matter in previous columns, but did you know it's "Dark Matter Awareness Week?" Of course you did. I'll celebrate by answering a question put to me by our own Charlie Jane Anders, who asks:

I'm curious about galaxies with dark matter cores, and how we can "see" dark matter.

I'll answer it even though she didn't frame it in the form of a question, especially since this is the sub-field that I actually work in.


Dark matter is everywhere. According to our best cosmological model, it makes up about around 25% of the energy in the universe, about five times as much as is found in the ordinary atomic stuff that makes up you, me, the sun and the earth. Dark matter is all around us, lurking stealthily.

What makes dark matter the ninja of the universe is that (by definition) it doesn't give off any light. However, Fritz Zwicky (feel free to share anecdotes about him in the comments section) noticed in the 1930's that there didn't seem to be enough mass to hold together galaxy clusters based on how fast the galaxies inside them seemed to be flying around.


But there's a long way to go between surmising the existence of dark matter and actually making a map of it, and for that, we need to really understand how gravity works. I should warn you in advance that the discussion's going to get a little hairy, but if you hold out, you'll be rewarded with pretty pictures at the end.

Just enough general relativity

Let me say just three things about how light and gravity work together. Actually, I'm going to say just one thing, but I'm going to say it three different ways, and with increasing complexity. Feel free to choose the one that you're most comfortable with, and go with that.

  1. Gravity bends lightbeams.

    Light is made of particles, and as they move past a cluster of galaxies, for example, they curve slightly towards it, just as a comet would as it passes by the earth. Yes, this produces a factor of two error. For this purpose, I can live with that. Clusters are big — they can contain hundreds or even thousands of individual galaxies, so the gravitational effect is pretty substantial. To my fellow physicists on site:

  2. Massive bodies really bend space-time, and light just takes the shortest route it can.

    Artwork by Starosta

    You've probably seen one of these diagrams before, in which space-time is imagined as a big rubber sheet with a marble in the middle of it. Stars, planets, and everything else, including light, make their way across this distorted sheet as best they can, occasionally taking convoluted routes.

    I'm not really crazy about this explanation because in reality, the bending of the time part of space-time is just as important as the space part, and the little diagram up there can't really show that. For those of you who are curious, including both terms is where the factor of two comes from.

  3. Light wants to take the fastest route possible.*

    General relativity fun fact: time runs slower near massive bodies than far away. So if a clever photon wants to get, say, from a distant galaxy to us and passes by a big cluster en route, making a slight detour makes sense. Even though light always travels at the speed of light, if time slows down locally, it takes light longer (according to people far away) to travel the same distance. Too close to the cluster, and time slows just enough to make the trip a hassle. Too big a detour, and the actual mileage becomes too much. There's a curved path around the edge that's just right.

    There actually may be several possible routes for light to take. This only happens with pretty extreme systems, and is known as "strong gravitational lensing." I'm not going to talk about that today, but it's also pretty neat because you get multiple images of the same galaxy.

    * Someone's going to take issue with me claiming to know what light wants. Really? That's the fight you want to pick?

All of this has been known and proven for over 90 years, since Sir Arthur Eddington observed the apparent positional shift of a star during the 1919 solar eclipse. Rather than a star where it was supposed to be in the sky, it was deflected a tiny amount away further away from the sun. That difference, about 1% the angle you could measure with your naked eye, was a big win for Einstein.

Credit: NASA

In other words, GR seems to be up to the task of predicting the deflection of light-beams.

Gravitational lensing

The big takeaway from this is that all types of matter, dark or ordinary, create a gravitational field, deflecting the path of light-beams. Though we're able to see ordinary stuff directly (because it gives off or absorbs light), with dark matter, we're not so lucky. To actually "see" dark matter, we need to use a technique called gravitational lensing.

Here's the setup: You and your telescope are here on earth (or in a nearby orbit), the thing you want to map out — a cluster of galaxies, for example — is somewhere far away in space, and there are lots and lots of very distant galaxies behind that. It's these distant galaxies that will help us out.

There's a catch, though. Unlike with Eddington's eclipse, we don't get to wait for a fortuitous event. The galaxies and clusters move very, very slowly, and so even if the light from the galaxies is deflected, we have no idea where they really would be if there weren't a big gravitational cluster in the way.

Light-beams from one side of one of the background galaxies get deflected by a different amount than light coming from the other side. The closer the light passes to the cluster, the more it'll be deflected; the farther it passes, the less it's deflected. This has the effect of pinching the image that we see. In other words, rather than looking circular, a galaxy would look to us like an ellipse. The more elliptical it is, the stronger the gravitational field, and the cool thing is that the effect will make the images of these galaxies tend to line up making a ring around the big masses in the cluster.

By the way, you've seen things like this in your everyday life. Just take an ordinary magnifying class, and look toward the edges. The ants that you're trying to fry will look oddly stretched and distorted.

The effect is typically pretty small (less than 1% for most images, though the effect can be huge for images in just the right place), so you need to average over hundreds of images to make a good map, but fortunately there are lots and lots of galaxies out there.

Go ahead and look at the picture at the top again. This is a very rich cluster called Abell 1689. You'll notice a couple of things. First, if you look really closely, you'll see that there are a bunch of little smudgy galaxies that are stretched into lines, and that they all seem to make a halo around the cluster (aka the big, bright galaxies in the middle). Incidentally, there are also hundreds of galaxies that you'd have a tough time detecting with your eye, but which can be picked out pretty easily with a computer program.

The cloudy bits in the image correspond to the reconstruction of where the mass is based on the lensing technique. Big note: You don't actually see this through the telescope. This is the matter we infer to be there based on how the background galaxies are distorted. This is also why "seeing" dark matter is in quotes. But I want to anticipate all the complaints down below. No doubt some cynic is going to say that since this is a reconstruction, it doesn't count. I want to point out, though, that the map we get isn't crazy no matter how you look at it. Remember, when we do the reconstruction, we don't look at the cluster galaxies themselves. But still, there is a lot of matter corresponding with each of them. That wouldn't happen if this method were straight-up bananas.

Not surprisingly, most of the stuff is near the middle. That's the nougaty dark matter core that Charlie Jane was asking about. You'll notice that a lot of the mass is lined up with the big bright galaxies (in gold). That's not a surprise. What is a surprise is that there are lots of regions which seem to contain little or no galaxies but which also contain a fair amount of matter.

NASA/CXC/STSci/CfA M.Markevitch et al., D. Clowe et al.

These maps can be made with all sorts of systems. Perhaps the most famous is from a few years ago of a system nicknamed the "Bullet Cluster," which is basically two rich clusters that have recently (or what passes for recently on cosmic timescales) collided with one another. The hot gas (in pink) has been stripped out and has a really cool shock front, but the dark matter from the two clusters (in blue) apparently just passed through one another. Since the vast majority of ordinary matter is in the form of gas, it seems kind of weird that the gas and mass don't seem to be lined up with each other.

This effect is not just limited to clusters, either. My friend, Richard Massey, and his collaborators made this cool 3-d view of the dark matter in one part of the universe by looking at lots and lots of background images at different distances. It's the same basic idea as measuring clusters, though. They're just measuring the shapes of lots and lots of little background galaxies.

Bonus question: How do we know dark matter isn't made of black holes?

I'd like to finish up with a mini bonus question that's closely related: How do we know that all of the dark matter isn't just black holes?

We can use lensing locally in our own galaxy as well as in the wider universe. While it's true that you can't see black holes directly, they do have strong gravity fields. Suppose there were enough star-sized black holes in our galaxy (besides the giant one in the middle) to make up the all the missing mass. Occasionally, pretty often actually, a black hole would pass in front of a background star. This would be just like passing a magnifying glass in front of the sun, only without the frying ants. For a few days (or weeks), the star appears brighter and then fades to the previous brightness in a predictable pattern. These "microlensing" events are pretty rare, but they do happen. All you have to do is monitor millions of stars and wait for one of them to brighten.

The juvenilely named MACHO and OGLE projects have done just that, and what they've found is that our Galaxy, at least, doesn't seem to have a giant swarm of black holes making up the missing mass.

So we still don't know what dark matter is, but at least we have some idea where it is.

Dave Goldberg would like to remind you that his book with Jeff Blomquist, "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty," is available on amazon, and would make a great present this holiday season. Follow him on twitter, facebook or his blog. He is an Associate Professor of Physics at Drexel University.