What is dark matter?

What is dark matter?

Dark matter doesn’t emit light. It can’t be directly observed with any of the prevailing tools of astronomers. Yet astrophysicists believe that it and dark energy structure most of the mass of the cosmos. What substance is, and what it isn’t. here.

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Airy-looking, cosmic-looking painting of long blue wisps, dotted with round black holes of varied sizes.
Since the 1930s, astrophysicists are trying to elucidate why the visible material in galaxies can’t account for a way galaxies are shaped, or how they behave. They believe a sort of dark or invisible matter pervades our universe, but they still don’t know what this substance could be . Image via ScienceAlert.

Dark matter may be a mysterious substance thought to compose perhaps about 27% of the makeup of the universe. what's it? It’s a touch easier to mention what it isn’t.

It isn’t ordinary atoms – the building blocks of our own bodies and every one we see around us – because atoms structure only somewhere around 5% of the universe, consistent with a cosmological model called the Lambda Cold substance Model (aka the Lambda-CDM model, or sometimes just the quality Model).

Dark matter isn’t an equivalent thing as dark energy, which makes up some 68% of the universe, consistent with the quality Model.

Dark matter is invisible; it doesn’t emit, reflect or absorb light or any sort of electromagnetic wave like X-rays or radio waves. Thus, substance is undetectable directly, as all of our observations of the universe, aside from the detection of gravitational waves, involve capturing electromagnetic wave in our telescopes.

Yet substance does interact with ordinary matter. It exhibits measurable gravitational effects on large structures within the universe like galaxies and galaxy clusters. due to this, astronomers are ready to make maps of the distribution of substance within the universe, albeit they can't see it directly.

They do this by measuring the effect substance has on ordinary matter, through gravity.

A flattened globe shape, with dark and lightweight blue and green patches fairly evenly distributed thereon .
This all-sky image – released in 2013 – shows the distribution of substance across the whole history of the universe as seen projected on the sky. It’s supported data collected with the ecu Space Agency’s Planck satellite. navy areas represent regions that are denser than their surroundings. Bright areas represent less dense regions. the grey portions of the image correspond to patches of the sky where foreground emission, mainly from the Milky Way but also from nearby galaxies, prevents cosmologists from seeing clearly. Image via ESA.

There is currently an enormous international effort to spot the character of substance . Bringing an armory of advanced technology in touch on the matter , astronomers have designed ever-more complex and sensitive detectors to tease out the identity of this mysterious substance.

Dark matter might contains an so far unidentified elementary particle of a kind completely different from what scientists call baryonic matter – that’s just ordinary matter, the things we see all around us – which is formed of ordinary atoms built of protons and neutrons.

The list of candidate subatomic particles breaks down into a couple of groups: there are the WIMPs (Weakly Interacting Massive Particles), a category of particles thought to possess been produced within the early universe. Astronomers believe that WIMPs might self-annihilate when colliding with one another , in order that they have searched the skies for telltale traces of events like the discharge of neutrinos or gamma rays. So far, they’ve found nothing. additionally , although a theory called supersymmetry predicts the existence of particles with an equivalent properties as WIMPs, repeated searches to seek out the particles directly have also found nothing, and experiments at the massive Hadron Collider to detect the expected presence of supersymmetry have completely did not find it.

Several differing types of detector are wont to detect WIMPs. the overall idea is that very occasionally, a WIMP might hit a standard atom and release a faint flash of sunshine , which may be detected. the foremost sensitive detector built so far is XENON1T, which consists of a 10-meter cylinder containing 3.2 plenty of liquid xenon, surrounded by photomultipliers to detect and amplify the incredibly faint flashes from these rare interactions. As of July 2019, when the detector was decommissioned to pave the way for a more sensitive instrument, the XENONnT, no collisions between WIMPs and therefore the xenon atoms had been seen.

Although WIMPs have long been the favored candidate for substance , they’re not the sole candidates. The failure to seek out WIMPs, and therefore the attendant frustration with not having the ability to account for a big percentage of the universe’s mass, has led many scientists to seem at possible alternatives.

At the instant , a hypothetical particle called the axion is receiving much attention. also as being a robust candidate for substance , the existence of axions is additionally thought to supply the answers to a couple of other persistent questions in physics like the Strong CP Problem.

Smiling old man in white shirt sitting down, pointing at something.
Astronomer Fritz Zwicky first predicted the existence of substance within the 1930s following his observations of the Coma galaxy cluster. Image via zwicky-stiftung.ch.

The idea that there could be things within the universe which are invisible to us, that emit no light, features a long history going back many years to the times of Newton. With the invention of so-called “dark nebulae” – clouds of interstellar dust blocking the sunshine from background stars – and Pierre Laplace’s 18th-century speculations about objects which could swallow light, later to become referred to as black holes, astronomers came to simply accept the existence of a so-called “dark universe.”

But in times , it had been astronomer Fritz Zwicky, within the 1930s, who made the primary observations of what we now call substance . His 1933 observations of the Coma Cluster of galaxies appeared to indicated it's a mass 500 times quite that previously calculated by Hubble . Furthermore, this extra mass appeared to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided an enormous piece of evidence for the existence of substance . She discovered that the centers of galaxies rotate at an equivalent speed as their extremities, whereas, of course, they ought to rotate faster. consider a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should always see in galaxies too. But we don't . the sole thanks to explain this is often if the entire galaxy is merely the middle of some much larger structure, as if it's only the label on the LP so to talk , causing the galaxy to possess a uniform rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is substance . Her ideas were met with much resistance from the astronomical community, but her observations are confirmed and are seen today as pivotal proof of the existence of substance . In honor of this significant and historic piece of detection toward establishing the existence of substance , the revolutionary Large Synoptic Survey Telescope, currently under construction in Chile and scheduled to ascertain dawn next year, was recently renamed the Vera C. Rubin Observatory.

A young female astronomer during a dress, performing at a telescope, surrounded by male astronomers.
Dark matter pioneer Vera Rubin (1928-2016). This image – taken at Lowell Observatory – is from 1965. Image via Carnegie Institute/ NPR.

Some astronomers have tried to negate the necessity the existence of substance altogether by postulating something called Modified Newtonian dynamics (MOND). the thought behind this is often that gravity behaves differently over long distances to what it does locally, and this difference of behavior explains phenomena like galaxy rotation curves which we attribute to substance . Although MOND has its supporters, while it can account for the rotation curve of a private galaxy, current versions of MOND simply cannot account for the behavior and movement of matter in large structures like galaxy clusters and, in its current form, is assumed unable to completely account for the existence of substance . that's to mention , gravity does behave within the same way in the least scales of distance. Most versions of MOND, on the opposite hand, have two versions of gravity, the weaker one occurring in regions of Low Mass concentration like within the outskirts of galaxies. However, it's not inconceivable that some remake of MOND within the future might yet account for substance .

Although some astronomers believe we'll establish the character of substance within the near future, the search thus far has proved fruitless, and that we know that the universe often springs surprises on us in order that nothing are often taken without any consideration .

The approach astronomers are taking is to eliminate those particles which can't be substance , within the hope we'll be left with the one which is.

It remains to be seen if this approach is that the correct