I had written previously about the blurry nature of the answer to the question "What is it that makes us human?" To me it seems like science, arts, and philosophy may be the three truly human activities. Had I been brave enough to pursue my "Teenage Dreams," I would have been a theoretical physicist, and rather than getting excited about what I read in popular science magazines, I would be working on these topics.
Lately, my favorite reading on the Internet are the 3 "Dark Mysteries" of cosmology. Let me introduce you to the three biggest issues that challenge our understanding of the universe:
This is the most "basic" problem, in the sense that it can be understood by employing basic Newtonian physics. If you ever had a long key chain, I am sure you did spin the key chain. Given a constant spinning force, the velocity with which the keys spin depend on the length of the chain, and the mass of the keys. It is not trivial to extend this analogy to spiral galaxies, but the rotational speed of the galaxies depend on the amount of matter they have. It has been observed in 1933 that the amount of observable material in galaxies is not enough to give them the speeds with which they are spinning. Thus there must be some type of matter that we do not know of, that does not interact with electromagnetic fields. The current estimations say %20 of universe is made up of this dark matter, which may be going through your body right now!
After finishing his work on Theory of General Relativity, Einstein modified his equations to include an anti-gravity term, which would keep the universe at a balance. He then rejected this as his biggest mistake.
However, it turns out that he may not have been wrong. By observing exploding stars far away from us, the astronomers are able to determine the rate with which they are moving away from us. Thanks to Erwin Hubble, it is a well established fact that the universe is expanding. However it was only after measurements of certain type of exploding stars were possible, that we were able to characterize this expansion. If we account only the dark matter, and ordinary matter (which is less then 3%) the expansion should be slowing down. But the fact is it is accelerating. So there must be some form of energy that works against gravity due to dark matter and ordinary matter.
So Einstein might have been right, when he thought he was wrong. The exciting world of Quantum Mechanics offer some explanations to this. One of the main principles of Quantum Mechanics is that one can never be certain about the velocity and momentum of a particle simultaneously -which is called the Heisenberg Uncertainty Principle. So you cannot be sure that the energy of vacuum is zero. This leads to the concept of virtual particles that come into and go out of existence in the vacuum, that may give some explanation to the dark energy that is pushing everything apart.
It is thought that 70% of energy/matter is due to dark energy.
This is the most interesting, and newest conundrum in cosmology. According to the standard model of cosmology the distribution of matter and energy in the cosmos is more or less uniform in large scales. By observing how remnant photons from Big Bang -which themselves are distributed evenly- are obstructed by groups of galaxies, the astronomies were able to find out how individual galaxy groups are drifting in the backdrop of the eternal expansion of the universe. According to the standard model they should be moving randomly. However it turns out that the observations tell a different tale of galaxies being pulled towards a certain point, without an apparent source of gravity to cause this pull.
What can be the reason?
To understand this one has to go into even more modern physics. The most accurate model of universe's evolution has an expansion phase just before Big Bang. During this expansion, the space-time expanded faster than the light itself. Some speculate that this expansion might be happening outside of "our" universe randomly, giving birth to different universes with fundamentally different characteristics. And some even go one step further in their speculations, and try to explain this Dark Flow by the gravity due to our sister universe(s)...
Science really makes us human.