She anticipated that the stars at the edge of the galaxy would move more slowly than those at its axis because the stars closest to the bright—and therefore massive—cluster of stars in the center would feel the most gravitational pull. However, she found that stars on the margins of the galaxy moved just as quickly as those in the middle.
Other astronomical observations have since confirmed that something strange is going on with the way galaxies and light move through space. Several experiments are searching for dark matter, and some of them may have even already found it. The problem is that no experiment has been able to make that claim with enough confidence to convince the wider scientific community—either due to statistics or an inability to rule out alternative possible explanations.
And no two claims have lined up quite convincingly enough for scientists to declare any result confirmed. The rate at which the experiment detected hits from possible dark matter particles changed over the course of the year—climbing to its peak in June and dipping to its nadir in December. This was exactly what DAMA scientists were looking for. If our galaxy is surrounded by a dark matter halo, the Earth is constantly moving through that halo as it orbits the sun—and the sun is constantly moving through the dark matter as it orbits the center of the Milky Way.
During half of the year, the Earth is moving in the same direction as the sun. During the other half, it is moving in the opposite direction. However, some loopholes exist; the particles the DAMA detector has been seeing could be something other than dark matter, something else the Earth and sun are constantly moving through.
Or something else could be changing in the nearby environment. It might be that people will come around only when several experiments start to see the same thing.
Dark matter could turn out to be something stranger or more complicated than we expect. In the space-based PAMELA experiment detected an excess of positrons—a possible result of dark matter particles colliding and annihilating one another.
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In the AMS experiment, attached to the International Space Station, found the same result with even more certainty. But scientists remain unconvinced, arguing that the positrons could also come from pulsars.
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It seems we will need to wait until the upcoming generation of dark matter experiments is complete to get a clearer picture. Scientists have come up with several models for what dark matter might be like. Other possibilities include particles conveniently already predicted in models of supersymmetry, a theory that adds a new fundamental particle to correspond with each one we already know. Groups of scientists are also searching for dark-matter particles called axions. Our dark matter story starts with speed and gravity.
Throughout the cosmos we see objects travelling in orbits under the influence of gravity. Just as Earth orbits the Sun, the Sun orbits the centre of our galaxy. The speed required to keep a celestial body in orbit is a function of mass and distance. For example, in our Solar System, Earth moves at 30km per second, whereas the most distant planets dawdle at several kilometres per second.
However, as we move further from the centre of the galaxy, the orbital speeds of the stars remains roughly constant. Read more: What a new map of the universe tells us about dark matter. Unlike our Solar System, whose mass is dominated by the Sun, mass in our galaxy is spread across thousands of light years.
Dark matter - Wikipedia
As one moves to larger distances from the galactic centre, the stars and gas enclosed within this radius increases. Can this additional mass explain the vast speeds of the most distant stars in our galaxy? Not quite. Back in the s, Swiss-American astronomer Fritz Zwicky found that galaxies orbiting within galaxy clusters were moving far faster than expected.
One possibility is that a vast amount of unseen mass extends beyond the stars and gas.
This is dark matter. Remarkably, our inability to see or detect dark matter provides clues as to how it behaves. It must have few interactions with itself and conventional matter apart from the force of gravity — otherwise we would have detected it emitting light and interacting with other particles.
As dark matter mostly interacts via gravity alone, it has some curious properties. A cloud of hot gas in space can lose energy by emitting light, and thus cool down. A sufficiently massive and cold gas cloud can collapse under its own gravity to form stars.
By contrast, dark matter cannot lose energy by emitting light. Thus, while conventional matter can collapse into dense objects like stars and planets, dark matter remains more diffuse. This explains an apparent contradiction. As the motion of dark matter is dominated solely by gravity, it is also comparatively easy to model analytically and in simulations. Since the s we have had formulae for the number of dark matter structures , which also happen to predict the number of massive galaxies and clusters of galaxies. That's a 1 with zeros after it. It's hard to get an answer that bad.
So the mystery continues. Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved.
Dark Matter's Biggest Problem Might Simply Be A Numerical Error
This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need? There are candidate theories, but none are compelling. The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.
What is dark matter? We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons.
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We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements.
But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS Weakly Interacting Massive Particles. Dark Energy, Dark Matter In the early s, one thing was fairly certain about the expansion of the universe. What Is Dark Energy? Universe Dark Energy-1 Expanding Universe. This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago.
The more shallow the curve, the faster the rate of expansion.