What is the universe made of? This is a seemingly simple question, but one might be surprised by the answer a cosmologist would give. We don't know! We can peer into the spectra of distance galaxies and stars and measure their chemical composition, and conclude that stars and gas clouds are made almost entirely of hydrogen and helium, with small amounts of heavier elements mixed in. The keyword here is 'chemical composition'. This refers to matter built from atoms of elements such as hydrogen and helium, carbon and iron. While planets, stars and people are built from atoms, the same may not be true for the whole universe. We now have good reason to think that the vast majority of the universe is built of a mysterious form of mass known as dark matter.
In the 1930's, Swiss astronomer Fritz Zwicky noticed that the galaxies in the Coma cluster of galaxies were not moving correctly under Newtonian gravity.They moved so fast, he found, that they should fly apart and the cluster should dissolve, according to Newton's Law's of Motion. He hypothesized that the only way that the Coma cluster can be kept together was if the cluster had hundreds of times more matter than could be seen by telescope. Zwicky, unfortunately, was ignored by the astronomical community, mainly because his hypothesis's deviated from the equations of Isaac Newton. 30 years later, Vera Rubin found the same anomaly while studying the motion of the Milky Way Galaxy. The same way that Mercury speeds around its orbit because it is close to the Sun, the inner regions of the galaxy should be moving faster. But this wasn't the case. Something outside the galaxy's disk was tugging on the stars. As the weight of her work and others began to mount, the astronomical community began to slowly be convinced of the missing mass problem. When Dutch radio astronomer Albert Bosma published the most complete analysis of dozens of spiral galaxies, and nearly all of them exhibited the same behavior, astronomers were finally convinced that dark matter did indeed exist.
While we may know that dark matter does exist, we are still in the dark as to what it is made of. Some scientists think that dark matter might be simply ordinary matter, just very dim, such as brown dwarf stars or neutron stars, which are nearly invisible. This matter is referred to as MACHO's, massive compact halo objects. Other scientists think that dark matter may consist of a very different matter than we are familiar with. Instead of being made up of protons and neutrons, it is made up of very hot matter, such as neutrinos. The leading candidate, however are WIMP's. WIMP's stands for weakly interacting massive particles. It's an entirely new type of matter, cold and dark, that we don't know much about at all.
Ordinary Galilean telescopes can't detect dark matter, whatever it may be. Using breaking edge telescope technology and lasers, astronomy has advanced to create instruments that help us make more sense of the universe. Confirming previous experiments, the WMAP satellite showed that the visible matter we see around us, mountains, planets, stars and galaxies, make up a small 4 percent of the total matter and energy content of the universe. According to the WMAP, the universe consists of 23 percent dark matter. The remaining 73 percent of the universe is made up of dark energy, which is even more mysterious than dark matter. Dark energy is completely undetectable. Dark matter is so abundant, that in our own Milky Way Galaxy dark matter outweighs all the stars by a factor of 10. Although invisible, dark matter can be detected, as it bends starlight and thus creates optical distortion.
As dark matter constitutes such a large portion of the universe, it has several heavy implications. The presence of dark matter could decide the fate of the universe, whether it will collapse, or continually expand and die in a big freeze. The fate of expansion of the universe has everything to do with mass. More mass means more gravity. And if there is enough gravity, it will be able to halt, and even reverse the occurring expansion of the universe. It was also the responsibility of dark matter for finally revealing the true age of the universe. The WMAP satellite, in its findings, showed the universe to be 13 billion years old. This rectified the very embarrassing previous estimation that the age of the universe was younger than the oldest stars detected.
The discovery of dark matter underlines the exotic nature of our universe. If such a large constituent of the universe was missing until 50 years ago, what else could our eyes be blind to? The future of astronomy will no doubt continue to dwarf and mesmerize us with it's exquisite complexity.