Dark Matter is embarrassing enough but at least it follows the laws of gravity. Dark energy seems to be doing the opposite!
Has it ever occurred to you that the main thing that happens when science advances is that we end up with more questions than we had before? For example, what is the stuff of the universe? Matter, at least matter as we know it, makes up only 4% of the universe. The rest is dark matter or dark energy and we barely know anything about them. How embarrassing!
It used to be a lot easier. The universe had matter and energy and then Einstein showed E=Mc2. That meant that matter could be turned into energy. This is how the sun creates energy, fusing hydrogen into helium and some leftovers to create energy.
Fritz Zwicky measured the mass of a galaxy cluster in the 1930’s. He noticed that the luminous matter in the galaxy cluster was far less than his measurements. Then, in 1959, Louise Volders noted that galaxy M33 does not rotate in a Keplerian manner. The speed of objects should be slower if the objects are farther from the center. In the 1970’s Vera Rubin found many galaxies that had this kind of rotation. The most likely reason for this was that there was more mass in the galaxy than was apparent particularly in the galaxy’s halo. In fact, if that extra mass isn’t there, the rotation of galaxies would pull them apart. This dark matter never heats up enough to radiate in the infrared so it is dark and it must be very cold. So this stuff is called cold, dark matter.
Dark matter turned out to be useful for describing how the universe could have structure like the galactic clusters when the Big Bang might otherwise have created a bland, homogenous universe. Many candidates for dark matter have been offered. The most popular is a particle called a Higgs boson. This particle would be much larger than a proton or neutron but it breaks down into smaller particles in the presence of heat. The Large Hadron Collider (LHC) in Europe may provide evidence that the Higgs boson is the right guy.
Unfortunately, while waiting for confirmation on the identity of dark matter, a bigger mystery appeared. Edwin Hubble studied the motion of galaxies and plotted their relative velocities on a graph. What he found was the further away the galaxy was the faster it was moving away from us. Why? If you drop a hundred marbles in the middle of a basketball court, the marbles that are furthest away must have higher velocities - at least in the first few seconds after the drop. Eventually even the furthest marbles would slow down and astronomers wanted to see if the galaxies were slowing down. If they were, then maybe the galaxies would once again collapse to a single point.
But astronomers found that the expansion of the universe was actually accelerating. Dark matter was embarrassing enough but at least it was acting like matter in one way, it had gravity. But whatever was causing this acceleration was doing the opposite. This other mystery component was given the name of dark energy. Dark energy itself is constant but it dominates as the universe expands and gravity becomes diluted.
It would be one thing if dark matter and dark energy were minor components of the universe. But the total mass of dark matter is probably 4 times the amount of normal matter. And dark energy makes up 5 times the amount of normal and dark matter combined. And to date, we can say very little about either one. How embarrassing!
[Ed.note: In mid-May 2010, a story appeared announcing that missing matter has been found. This missing matter is normal or baryonic matter found in intergalactic space and discovered in X-rays. This matter is also called WHIM (Warm-hot intergalactic matter). This does not change the current estimates of dark matter or dark energy. See http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=47026 and http://www.sciencecodex.com/xray_discovery_points_to_location_of_missing_matter
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