For something that\u2019s literally as old as the universe, dark matter doesn\u2019t get much attention outside scientific circles. Maybe that\u2019s because, other than a short-lived SyFy series and a late-period Randy Newman album, this nebulous star stuff has had a tough time breaking the pop-culture barrier.But the truth is that today, dark matter has never mattered more. Our own Milky Way is embedded in a massive cloud of it, we\u2019re looking for its interactions deep inside the earth, and there are whole galaxies without it.So what is dark matter, anyway? Why can\u2019t scientists get enough of the stuff, even though they can\u2019t actually find it? What deep, dark secrets does it hold? And could it ultimately shape the future of life as we know it?Why Is Dark Matter a Mystery?The history of dark matter shows just how elusive this star stuff can be.Physicists first theorized the existence of dark matter in 1933, partly because equations showed that there wasn\u2019t enough observable matter in galaxies to keep them from disintegrating, and observed rotational speeds of galaxies didn\u2019t fit the expected results from standard physics models.Further research largely waited until the 1970s, when better scientific instruments, from receivers to gamma radiation-detecting space telescopes, let astronomers and physicists confirm the earlier calculations and observations. Powerful radio telescopes also offered clues like gravitational lensing (where matter causes light to bend between its source and the observer) and strongly suggested that there was a kind of matter out there we could detect, but not see. \u201cEverything you can see, everything you feel, everything you\u2019re made up of, only makes up 5 percent of the universe, and the rest is this dark stuff\u2026and we have no idea what it is,\u201d says Rebecca Leane, a theoretical physicist at MIT. Leane\u2019s Ph.D. dissertation was on the phenomenology of dark matter.Physicists estimate some 27 percent of the total universe is dark matter and the rest (68 percent) is a similarly shadowy phenomenon called dark energy. What makes dark matter so mysterious?\u201cThe big thing about it is that we can\u2019t see it; it doesn\u2019t interact with light,\u201d says Ethan Brown, an Assistant Professor of Physics at Rensselaer Polytechnic Institute. (The photo at the top of this article is a composite image from optical and x-ray telescopes where the blue shading depicts the likely dark matter, even though it doesn't show up directly in the images.)Broadly, we can measure matter and energy in the universe by observing it in one of four interactions:With electromagnetic radiation (a.k.a. light)Through gravitational effectsWith other matter through the strong nuclear force, which holds matter togetherWith the weak nuclear force, or the interaction of subatomic particles that\u2019s responsible for radioactive decayDark matter eludes most of those observations because it doesn\u2019t appear to interact with standard matter at all, except through gravity. But that hasn\u2019t stopped physicists from ruling out other methods.One of Brown\u2019s areas of study is trying to capture dark matter interactions with normal matter in the form of liquid xenon isotopes. Xenon-124 has a half life roughly a trillion times longer than the age of the universe. Massive vats of the stuff are tucked deep into boreholes in the earth's crust to limit background noise like electromagnetic radiation that could interfere with measurements. Only dark matter and certain subatomic particles like muons and neutrinos can pass through the thousands of feet of dense rock. So it's a very "quiet" room, where\u2014theoretically\u2014only Xenon-124's exceptionally slow natural radioactive decay, or interactions with muons, neutrinos, or dark matter could cause some kind of change in the isotope. If a subatomic particle of dark matter knocks out an electron from the Xenon-124, the thinking goes, the Xenon1T experiment will see it.While dark matter scientists haven\u2019t actually detected direct interactions with the elusive subatomic particles yet, they\u2019ve certainly made some other interesting observations\u2014including the decay of xenon-124, only the rarest event ever recorded in human history. So What Is Dark Matter?We know more about what dark matter isn\u2019t than what it is. For starters, it isn\u2019t dark energy. That\u2019s some kind of energy for which the evidence is also indirect, but likely exists because the universe is expanding at an increasing rate of speed, which defies the laws of physics of normal matter and energy.And dark matter isn\u2019t antimatter, either, which is normal matter composed of subatomic particles that have an exact opposite charge to matter. When antimatter and matter collide, the annihilation produces bursts of gamma rays. Dark matter can also produce gamma rays when it and its counterpart, dark antimatter, collide to produce standard matter.And finally, dark matter isn\u2019t just a different class of the three families of ordinary matter like hadrons, leptons, or bosons, the latter two of which were formerly theoretical, but have finally been directly observed in particle accelerators and don\u2019t behave like we expect.Leptons and bosons do give us a hint to follow, however. Dark matter appears to be a form of matter made up of an entirely different class or classes of subatomic particle. One of the most promising is called a WIMP: a weakly interacting massive particle. WIMPs, despite their puny name, are thought to have a mass as much as a thousand times more than standard matter\u2019s protons. And the way that WIMPs are theorized to work fits neatly with calculations of how much dark matter there must be in the universe, says Leane. This is called the WIMP Miracle.But WIMPs are far from the only theory in play. There are also primordial black holes, which are essentially small black holes left over from the Big Bang. However, we haven\u2019t observed gravitational microlensing from them, so that rules out some masses of primordial black holes as possible dark matter.Then there are theorized particles like SIMPs and axions\u2014and countless other potential clues. \u201cThere are more theories out there now than I\u2019ll ever understand,\u201d Brown admits.Naturally, it can be pretty annoying to do research when you can\u2019t actually observe something you think exists, or isn\u2019t always there. Researchers at Yale, for example, have found two galaxies that don\u2019t have any dark matter at all.\u201cIt\u2019s hard to point out just one solution for how these might have formed,\u201d says Shany Danieli, a doctoral student at Yale who co-authored two of the studies. \u201cAt the beginning, we thought maybe it was just some kind of anomaly, but now we found a second galaxy.\u201dThe research points to some fascinating possibilities for how dark matter functions in the universe: that dark matter interacts with normal matter via a mechanism that we don\u2019t yet know\u2014a so-called \u201cdark force,\u201d or fifth force in the universe. Another idea is that dark matter does interact via more of the known forces than just gravity, but does so at such a tiny interaction strength that we simply don\u2019t have the means yet to reliably detect the signals.In other words, the science is far from conclusive.What Dark Matter MeansWhy, then, are physicists so focused on unraveling the mystery of dark matter? \u201cThe work of particle physics the past 50 years has been to break down the universe to its smallest components,\u201d says RPI\u2019s Brown.Right now, dark matter doesn\u2019t fit certain understandings of how the universe works, in particular the standard model of particle physics. \u201cWhen we understand what dark matter is, and how it behaves, that\u2019s a huge step to understand the fundamental underpinnings of the universe,\u201d Brown says. \u201cWe can answer questions like how did the universe develop to what it is today?\u201dPlus, fundamental particle physics, including the search for dark matter, has already produced real technological gains. Many of the detection tools used in the field are highly applicable to other areas like medical imaging or nuclear security. Leane points out that the internet was created in part because particle physicists at CERN wanted to find new ways to share data with each other. GPS, meanwhile, relies in some measure on Einstein\u2019s theory of general relativity, which explains how gravity curves space and time, says Danieli. We can\u2019t begin to know what might emerge from our study of dark matter, but consider an analogue from the study of conventional matter, which yielded the most fundamental technology that allows us to do practically anything in modern life. Without J.J. Thomson\u2019s discovery of the electron in 1897, we wouldn\u2019t even have electricity, much less computers and an internet powered by it. So while we still don't know much about dark matter today, it could very well change the way we live tomorrow.