What is dark matter? Dark energy? How were they discovered and what do they do? What is their importance to physics and astronomy? These are some of the questions that I will answer in this essay.
In the early 1990’s one thing was fairly certain, that the Universe would either collapse due to energy density or keep expanding forever; but gravity should inevitably slow the expansion down. Well, they were wrong. In 1998 the Hubble Space Telescope (HST) changed theories and created more speculation. The Universe was actually expanding faster than thought and was speeding up. This conclusion came from observing super novae from closer to the time of the big bang. Meaning the universe was expanding slower, billions of years ago.
The solution to this conundrum was Dark Energy; more is unknown than is known about dark energy. They know how much dark energy there is because of the way it affects the universe expansion, but that’s about it. Roughly, 70% of the universe is made up of dark energy,
25% is dark matter and only about 5% of the universe is what we can actually see and have observed. Considering how mind boggling everything we can see already is, that’s not even close to the amount that is really there. This led to Albert Einstein’s theory of Cosmological
Constant.
This consisted first of him realizing that empty space is not nothing, and that it’s possible for more space to come into existence. The second prediction

says that “empty space” can posses its own energy. This is just one of many theories about the
Universe. Unfortunately, there are more theories and speculation than there are answers.
Dark matter is also another form of matter in physics and astronomy that is not understood very well. Scientists are more certain of what dark matter is not than they are about what it is. They know it’s dark, meaning it’s not in the form of regular matter that we can see and touch; they know it comprises about 25% of the Universe. Second, dark matter is not anti-matter because we do not see the gamma rays which are emitted when matter and antimatter collide. Finally, dark matter is not galaxy sized black holes, because of the amount of gravitational lenses we see; which I will discuss in more depth later on. There seems to be two main categories of objects that scientists could consider as possibilities for dark matter. These are MACHO’s (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive
Particles).
MACHOs are objects ranging in size from small stars to super massive black holes.
MACHOS are made of ordinary matter (like protons, neutrons and electrons). They may be black holes, neutron stars, or brown dwarfs.
Astronomers have been detecting MACHOs using their gravitational effects on the light from distant objects. In formulating his theory of gravity,
Einstein discovered that the gravitational attraction of a massive object can bend the path of a light ray, much like a lens does. So when a massive object passes in front of a distant object like a star or another galaxy, the light from the distant object is "focused" and the object appears brighter for a short time. This is called the “gravitational lense” technique. This was also later confirmed by Fritz Zwickie. Theorists speculate that a lot of the dark matter could be tied up in brown dwarfs; which are stars with a mass that is less than 8% of the sun and therefore has to little mass to have the nuclear reactions that make the stars shine. Unfortunately, scientists have not found a great enough population of brown dwarfs to account for the 25% of the universe made up of dark matter.
WIMPs are the subatomic particles which are not made up of ordinary matter. They are
"weakly interacting" because they can pass through ordinary matter without any effects. They are "massive" in the sense of having mass (whether they are light or heavy depends on the particle). The prime candidates include neutrinos, axions, and neutralinos. Neutrinos were first
"invented" by physicists in the early 20th century to help make particle physics interactions work properly. They were later discovered, and physicists and astronomers had a good idea how many neutrinos there are in the universe; but they were thought to be without mass. This thought changed in 1998; one type of neutrino was discovered to have a mass, albeit very small. This mass was too small for the neutrino to contribute significantly to the dark matter.
Axions are particles which have been proposed to explain the absence of an electrical dipole moment for the neutron. They thus serve a purpose for both particle physics and for

astronomy. Although axions may not have much mass, they would have been produced abundantly in the Big Bang. Neutralinos are members of another set of particles which has been proposed as part of a physics theory known as super symmetry. This theory is one that attempts to unify all the known forces in physics. Neutralinos are massive particles (they may be 30x to 5000x the mass of the proton), but they are the lightest of the electrically neutral super symmetric particles. The evolution of structure in the universe indicates that the dark matter must not be fast moving, since fast moving particles prevent the clumping of matter in the universe. So while neutrinos may make up part of the dark matter, they are not a major component. Particles such as the axion and neutralino appear to have the appropriate properties to be dark matter. However, they have yet to be detected.
Dark Matter is also a theory of what is causing the galactic rotation problem. Vera Rubin is famous for uncovering the discrepancy between the predicted angular motion of galaxies and the observed motion, by studying galactic rotation curves; calculations of how a galaxy should move are different than what is observed. This discrepancy is currently thought to betray the presence of dark matter that permeates the galaxy and extends into the galaxy's halo. The halo is a region at the outer edges of a galaxy, best seen in spiral galaxies. This halo is what extends beyond the main visible components of the observable galaxy. The most accepted theory of how these structures came to be is that all the large-scale structure of the cosmos we observe today was formed as a consequence of the growth of the primordial fluctuations, which are small changes in the density of the universe in a confined region. As the universe cooled clumps of dark matter began to condense, and within them gas began to condense. The primordial fluctuations gravitationally attracted gas and dark matter to the denser areas, and thus the seeds that would later become galaxies were formed. These structures constituted the first galaxies. This brought about galactic clusters which are big groupings of many galaxies all being held by a mutual gravitational force.
However, their velocities are too large for them to remain

gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. One idea is black holes and the other is dark matter. This picture of a galaxy with a super massive black hole at its center is what holds the galaxy together.
In cosmology, Cosmic Microwave Background Radiation (CMBR) is thermal radiation which fills the observable universe almost uniformly. With a traditional optical telescope, the universe background is completely dark. When using highly sensitive telescopes specifically in the microwave region, the universe exhibits a faint background glow almost perfectly uniform in all directions. Cosmic background radiation is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. The big bang was what scientists call the creation of our known universe. When our universe was very young and small, it consisted of all the matter and energy we know of today. It was also extremely hot. After the big bang, and things started to cool and electrons, protons and atoms started to form as the white hot plasma and radiation cooled and expanded. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. This is a map of the known universe and its’ CMBR.

The universe is estimated to be about 13.75 billion years old and has continues and increases the rate at which it is expanding. This leaves many scenarios for the fate of the universe and most depend on the density of the universe.
In conclusion to the theories and relevancy of dark matter and dark energy; there is much more to learn and understand about the universe and how it works. The study of dark matter and dark energy is still ongoing and in its infancy but as scientists learn and discover more about our universe, they will be able to answer more of the questions they have.