I listened to a course on the existence of dark matter and dark energy in the universe. It was discovered by Edwin Hubble in 1929 that the universe is expanding. The expansion needs energy to propel it, it is thought, but the energy required, while actually present and creating the expansion, is greater than can be accounted for the total of all known matter in the universe. Therefore, scientists are forced to postulate something else in addition to the matter and energy that are known. As research and thinking have proceeded, evidence required two postulations, one of dark matter and one of dark energy. Calculations show that given the known facts about the universe, 75% of it must be this mysterious dark energy and 20% must be dark matter. That leaves on 5% of the universe to consist of all the known matter in all the hundreds of billions of galaxies, including ours, our solar system, our planet and us.
As I watched Prof. Sean Carroll in his Teaching Company DVD course, I heard him repeatedly refer to heat as the major indicator of energy. I have heard of the heat death of the universe, a time when all "warmth" has flowed into all "cold" and everything is at the same temperature, which I think I have heard him say is estimated to be at something like 3 degrees Kelvin or 457 degrees below zero on the Farenheit scale. I have read that the concept of the heat stasis of universe was first considered in the 1850's and might not be all that relevant to today's thinking about dark energy and dark matter.
If you are ready to do some web surfing sometime, all the topics in red are current areas of research and thinking. You can read about them on many web sites.
