Dark Matter and Dark Energy

What are dark matter and dark energy, and how are they affecting the universe?

Our universe may contain 100 billion galaxies, each with billions of stars which can produce an abundance of energy. Yet everything that we can see in our universe is like the tip of the cosmic iceberg — it accounts for only about four to five percent of the total mass and energy in the universe. Only four percent of the universe is made of materials we sort of understand. So what about that remaining 96%?

For the most part we’ve labeled it under two names, dark matter and dark energy. We have no clear idea what these materials are.

The Milky Way (our galaxy) is shaped like a disk that is about 100,000 light-years across. The stars in this disk all orbit the center of the galaxy. The laws of gravity say that the stars that are closest to the center of the galaxy — which is also its center of mass — should move faster than those out on the galaxy’s edge. 

Yet when astronomers measured stars all across the galaxy, they found that they all orbit the center of the galaxy at about the same speed. Calculations show that a vast “halo” of dark matter surrounds the Milky Way. The halo may be 10 times as massive as the bright disk, so it exerts a strong gravitational pull. 

The same effect is seen in many other galaxies. And clusters of galaxies show exactly the same thing — their gravity is far stronger than the combined pull of all their visible stars and gas clouds.

  • Dark energy and dark matter describe proposed solutions to as yet unresolved gravitational phenomena. So far as we know, the two are distinct.
  • Dark matter originates from our efforts to explain the observed mismatch between the gravitational mass and the luminous mass of galaxies and clusters of galaxies.
  • The gravitational mass of an object is determined by measuring the velocity and radius of the orbits of its satellites, just as we can measure the mass of the sun using the velocity and radial distance of its planets.
  • The luminous mass is determined by adding up all the light and converting that number to a mass based on our understanding of how stars shine.

How does dark matter affect the universe?

  • Dark matter must be the basic building block of the largest structures in the universe: galaxies and clusters.
  • Without dark matter, the universe would be a very different place, according to current theories.

Dark energy, on the other hand, originates from our efforts to understand the observed accelerated expansion of the universe.

  • In a nutshell, current theory cannot explain the acceleration.

One speculative possibility is that the acceleration is a consequence of another new form of matter, nicknamed dark energy, which has hitherto gone undetected.

Because of the names, it’s easy to confuse dark matter and dark energy. And while they may be related, their effects are quite different.

About one-quarter of the universe consists of dark matter , which releases no detectable energy, but which exerts a gravitational pull on all the visible matter in the universe.

  • In brief,  dark matter attracts, dark energy repels. While dark matter pulls matter inward, dark energy pushes it outward.
  • While dark energy shows itself only on the largest cosmic scale, dark matter exerts its influence on individual galaxies as well as the universe at large.
  • Are dark matter and dark energy related? No one knows. The leading theory says that dark matter consists of a type of subatomic particle that has not yet been detected. Dark energy may have its own particle, although there is little evidence of one.
  • Instead,  dark matter and dark energy appear to be competing forces in our universe. The only things they seem to have in common is that both were forged in the Big Bang, and both remain mysterious.

Is Dark Matter & Dark Energy the Same Thing?

Dr HongSheng Zhao, of the University’s School of Physics and Astronomy, found that the puzzling dark matter and its counterpart dark energy are so closely intertwined that it’s not clear that they’re even two different materials.

In Dr Zhao’s model,  dark energy and dark matter are simply different manifestations of the same thing, which he has considered as a ‘dark fluid’. On the scale of galaxies, this dark fluid behaves like matter and on the scale of the Universe overall as dark energy, driving the expansion of the Universe. Efforts are currently underway to hunt for very massive dark-matter particles with a variety of experiments.