“Time takes it all whether you want it to or not, time takes it all. Time bares it away, and in the end there is only darkness.” -Stephen King

But we are not quite at the end of time yet! It’s only the end of the week, which means it’s time for another Ask Ethan, and to give away another 2015 Year In Space Calendar! After another great week of questions and suggestions (and there were many good ones), congratulations are in order for last-minute submitter Joe Latone, who asks about a newly released story:

I’m seeing a lot of physics headlines like this over the past day, Researchers detect possible signal from dark matter. As you so eloquently do, would you explain a bit of the background and then distill this recent news for us?

Let’s give you exactly what you want and need, Joe!

First off, there’s the problem of dark matter. When we think about a cluster of galaxies — like the Coma Cluster, above — we have two ways of measuring the stuff that’s in it:

We can look at the full spectrum of signals from the electromagnetic spectrum coming from it, including not only the light-emitting stars but also light emitted and absorbed from other parts of the spectrum. These give us windows into the amount of gas, dust, plasma, neutron stars, black holes, dwarf stars and even planets present inside. We can look at the motion of the objects within the cluster — in this case, the individual galaxies — and use what we know about the laws of gravitation to deduce what the total amount of mass within is.

By comparing those two numbers, we can see whether all of the mass is accounted for by normal matter, or whether there needs to be something else that isn’t made of protons, neutrons and electrons.

Image credit: Multiwavelength images of M31, via the Planck mission team; ESA / NASA.

We can do the same thing for individual galaxies as well. Again, it’s easy to look at all the different, multiwavelength components of the galaxy. For both individual galaxies as well as clusters, we find a certain amount of mass in the form of stars, about five-to-eight times as much in the form of neutral gas, very little in the form of plasma (although there’s plenty of plasma in the intergalactic medium), and only a fraction of what’s present in stars in the form of all the other types of mass, combined. On average, there’s about seven times as much total normal matter in addition to the stars we see in all the large galaxies and clusters we look at.

But when it comes to the total amount of mass that we infer from gravitation, we find something surprising. Rather than needing about eight times as much total matter to account for the gravitational effects we see, which are the rotational speeds of galaxies at different distances in individual spirals and the speeds of the individual galaxies relative to the cluster center in clusters, we need something like fifty times as much!

This discrepancy, or the fact that we need about a total of five times as much matter in addition to the amount of normal matter that exists in our Universe, is known as the dark matter problem. There are many good sets of observations — including from distance/redshift measurements of standard astronomical candles, from giant surveys of the large-scale structure in our Universe, from observations of colliding galaxy clusters and from precision measurements of the Cosmic Microwave Background (the leftover glow from the Big Bang) — that show this is not a problem with the theory of gravity itself, but is rather due to the fact that there is a new type of matter in our Universe that exists in about five times the abundance of normal, atomic matter.

And this new form of matter — dark matter — among other things, does not interact with either matter or radiation through the electromagnetic force.

Image credit: The Particle Adventure / DoE / NSF / LBNL, original from CPEP via http://cpepweb.org/.

It’s also been established that whatever this dark matter is, it isn’t any of the conventional particles in the standard model. It isn’t a quark, it isn’t a boson, and it isn’t even a neutrino. Whatever it is, it’s got to be an entirely new type of particle, one that hasn’t been discovered yet.

Based on the gravitational properties that it’s required to have, as well, it’s expected to cluster in a giant halo, both around galaxies individually and around huge clusters in even larger, more diffuse spheroids.