 We have seen two kinds of evidence for dark matter. One from galaxy rotation curves for dark matter around spiral galaxies, and the other from gravitational lensing for galaxy clusters. Both leave us with only two possibilities. Either our current theory of gravity just doesn't extend to galaxies and or galaxy clusters, or most of the universe is made up of one or more unknown substances that interact with normal matter via gravity, but not much else. A tremendous amount of research is actively trying to find out what this stuff is. Here are some of the possibilities. Hydrogen, weakly interacting massive particles, wimps for short, neutrinos. Let's take a quick look at each of these. Here's a European Southern Observatory artist's representation of the distribution of dark matter around the Milky Way Galaxy. You may recall from our How Far Away Is It segment on the Milky Way that in September of 2012, Shandra found evidence that the Milky Way Galaxy is embedded with a large amount of hot gas in the halo. Counting this vast amount of gas, the mass of the halo is estimated to equal the mass of the star's new galaxy. This could be the solution for star orbital speeds and might be the answer for spiral galaxy dark matter. It could not be the answer for galaxy clusters though, but then again, it doesn't have to be. Another possibility is the existence of a new set of extended standard model particles. There are a variety of theories that predict wimps. In late 2015, the Large Hadron Collider at CERN came back online after a two-year upgrade. It can now reach 13 trillion electron volts, almost double the collision energy before the upgrade. At this level, it might be enough to produce a wimp. Of course, the particle would not be detected because it wouldn't interact with any layer of the detector. However, it would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum missing after a collision. You may remember from our How Small Is It? elementary particle segment that this is exactly the same way Ellison Wooster found the neutrino back in 1927. This brings us to the last possibility we'll cover. In our discussion on elementary particles, we found that the neutrino is a critical component in many nuclear reactions. And the detection of solar neutrinos and of neutrinos from supernova 1987A marked the beginning of neutrino astronomy. Over the life of the universe, countless numbers have been produced. There are over a billion times more abundant than the electrons, protons, and neutrons that make up stars, planets, and people. But are there enough of them to account for the dark matter in galaxy clusters? Neutrino astronomy is trying to discover the answer to this question. Today we have built amazing neutrino detectors, such as the Super Kamiokande in Japan, to better understand these fundamental but elusive particles. The Super Kamiokande is located 1,000 meters underground in a Japanese mine. It contains a lake holding 50,000 tons of ultra-pure water surrounded by an inner detector with over 11,000 photomultiplier tubes, a flash when struck by a photon created by a neutrino interaction with the water. The speed of light in water is slower than the speed of light in a vacuum. A neutrino interaction with the electrons or nuclei of water can produce a charged particle that moves faster than the speed of light in water. This creates a cone of light known as Chernikov radiation. This is the optical equivalent of a sonic boom. The Chernikov light is projected as a ring on the wall of the detector and recorded by the photomultipliers. With so much of the scientific community searching for an answer, we can expect to know if neutrinos, wimps, hydrogen, new gravitational theories or something else can explain the movement of stars around galaxies and galaxies around clusters. With all this activity, I expect we'll hear a lot more about neutrinos in the coming years.