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I have been studying Physics for approximately four months, and if I have learnt anything through my lectures and reading, it is that there are STILL some major problems with our comprehension of the universe that we exist in. Before my studies I (somewhat naively perhaps) assumed that after the existence and findings of great minds such as Aristotle, Galileo, and Einstein, that we’d pretty much figured everything out by now, and that a modern day physicist’s job was simply to tie up the loose ends left over, and clean up. I have since learnt this to be far from the truth, and that we have only just scratched the tip of the iceberg of scientific knowledge. One term that seems to have been popping up in the media a lot recently, and once or twice in our astrophysics course is DARK MATTER. The term intrigued me, as it sounded quite sinister and sexy. So, when I learnt that I had to do an oral presentation on something scientific, I thought it was a good opportunity to find out what this DARK MATTER is, and today I am going to try and tell you guys a little bit about it. Firstly, the term DARK MATTER is a loose one, referring to a substance that does not emit or reflect enough electromagnetic radiation to be detected directly. In other words, with our current technologies, we cannot see this matter, and thus the term dark is merely a reflection of our lack of understanding of the matter, rather than any reference to it’s colour. So if we cannot see this so called DARK MATTER, how do we know it’s there? Well, the existence of DARK MATTER can be inferred from observational evidence of the motion of galaxies. One example of this is the motion of a spiral galaxy, rotating in space. Newtonian and Keplerian mechanics suggest that the velocity of an orbiting star is governed by the same respective laws that govern orbits in our own solar system. These laws state that the velocity of the orbiting planet (say Earth) is proportional to the mass of both the Earth and the object that the Earth is orbiting (in this case the Sun), as well as being inversely proportional to the distance between the two objects. Thus if you increased the distance between the Earth and the Sun, you would expect it’s orbital speed to decrease (hence Neptune has a slower orbital velocity than Earth). Now, if we return to our spiral galaxy, we would expect an orbiting star to behave in a similar way. HOWEVER, careful observational evidence has proven this NOT to be the case, and that in-fact orbiting stars in such circumstances actually travel more or less uniformly regardless of their radius, and at a much faster velocity than predicted by the mathematics based on the amount of visible mass. THUS, for such a star to maintain a high velocity orbit, there must be something else out there, having a strong gravitational effect on the star’s orbit…This indicates the existence of the mysterious DARK MATTER. To explain the phenomena observed, we must rethink our model of the universe to include this dark matter. One way of visualising this is to imagine the dark matter as the skeleton on which the flesh of the universe is hung. This indicates that all visible matter (including the earth) is surrounded by dark matter. In-fact, cosmologists now believe that the MAJORITY of the universe is composed of DARK MATTER, with the visible part only making up approximately 10% of what is out there! I find this quite a startling concept, as we often look up to the heavens in amazement, but really it is just some minor detail in the grand scheme of things, and it makes ones life and problems seem particularly insignificant! So, if we are to accept the existence of DARK MATTER, than what actually is it? Well, we don’t actually know, although there are a number of competing theories in cosmology which are yet to be experimentally proven. Some believe that the dark matter is composed of baryonic matter (i.e. protons, neutrons, and electrons, the same stuff that the visible matter is made of), and some believe that it is made of more exotic non-baryonic particles which are yet to be discovered. Of the baryonic matter, the belief is that the dark matter comes from massive dark bodies, such as black holes, which do not emit light due to their strong gravitational pull, and failed stars such as brown dwarfs. This type of dark matter is collectively known as MACHOs which stands for MASSIVE COMPACT HALO OBJECTS, but calculations suggest that they can only account for up to 20% of the predicted dark matter in the universe. Thus the rest must be composed of non-baryonic matter. This non-baryonic matter is referred to as WIMPs which stands for WEAKLY INTERACTING MASSIVE PARTICLES, and is currently believed to contribute up to 90% of the dark matter in the universe. WIMPs are theoretical particles, and have never been detected. If they do exist, they are extremely difficult to detect with current technologies as they do not interact with electromagnetism or the strong nuclear force, and thus cannot be seen. There are currently numerous experiments being conducted throughout the world in an attempt to discover WIMPs. Many of these experiments are conducted deep underground, as it is believed that their detection is hampered by interference from cosmic rays; WIMPs are believed to be incredibly strong and passing through the Earth all the time, whilst cosmic rays are halted by the atoms in the rock, making a deep mine the ideal place to try and detect a WIMP. So Why does this all matter? Does Dark Matter, Wimps, Machos, (not to mention DARK ENERGY, and NEUTRINOS amongst others) etc have any significance in the long run? Well the answer is yes, and one extremely significant role they play is in aiding us to predict the fate of our universe. At the moment there are two competing theories of the end: The Big Crunch, and The Big Freeze. The Big Crunch is where the expansion of the universe slows down and is overcome by the attractive force of gravity. All the matter in the universe will be drawn together in an almighty black hole. The Big Freeze is where the universe continues to expand and eventually becomes too cold to sustain life. Now, these two fates are governed by the density of the universe, and thus if we have a better understanding of the nature of dark matter, we can calculate the density of the universe more accurately, and therefore make a more precise prediction of how it will all end… Well, thanks for listening to me, and if you have any questions I will do my best to answer them.