Dark Matter vs. Dark Energy
Difference Between Dark Matter And Dark Energy
Scientists calculate that around 22 percent of the universe is dark matter. It does not form stars. It does not emit light. But it exerts gravity in much the same way as ordinary matter does. That is how astronomers know it is there. They can observe dark matter’s pull on visible objects. It affects the motion of stars. It even affects the motion of entire galaxies. These observations allow scientists to calculate the enormity of dark matter in the universe. But they still do not know what dark matter is.
At first, astronomers thought dark matter consisted of nonstellar matter. This would include interstellar dust and planets. Then, in the 1980s, astrophysicists demonstrated that dark matter could not contain protons and neutrons. They determined this based on theories about the formation of chemical elements in the early universe. They concluded that dark matter must be nonbaryonic.
Other physicists proposed that two kinds of particles might make up dark matter: superpartners and axions. Superpartners are predicted by supersymmetry theories. According to these theories, large superpartners decay into lighter ones. But the lightest ones are stable. The lightest known superpartner is LSP (Lightest Supersymmetric Particle). LSP particles would have formed during the Big Bang. Moreover, they would survive even today. So they might account for much of the universe’s dark matter.
Axions would be lighter than LSPs. They are also predicted by various mathematical theories. If axions and LSPs make up dark matter, they would be all around. So the right type of experiment should reveal their presence. To this end, physicists continue to build devices aimed at detecting these dark particles.
About 74 percent of the universe may be dark energy. As yet, scientists do not have a theory explaining the origin of dark energy. But they have observed its effects in experiments. They have also described some of its properties. For instance, dark energy does not emit light. In addition, it does not clump together. Instead, it spreads uniformly through the universe. (By contrast, dark matter and ordinary matter clump into galaxies.) Third, dark energy does not exert gravity. Quite the opposite, it would cause the universe to expand at an increasing rate.
Evidence for the existence of dark energy is increasing. It includes the recent, surprising discovery that the universe is, indeed, expanding. As the universe expands, baryonic and dark matter will decrease in density. But dark energy either does not decrease in density or does so very slowly. So in the future, dark energy will make up more of the universe than it does now. Many scientists are eagerly planning experiments to learn more about the properties of dark energy.