Dark MatterDark Matter is invisible, but has been postulated from its apparent influence on normal matter. It is one explanation for the lack of gravity needed to hold our universe together. (The current estimations of the total mass in the universe wouldn't support the observations and given the current numbers; our star systems would simply fall apart.) The dark matter theory states there is matter that doesn't radiate and therefore we cannot observe, as all of our methods rely on picking up electromagnetic waves of some sort.
This is the observation that has thrown everyone off guard. There is a certain type of supernova called type Ia. The nice thing about type Ia supernova is that they all seem to have the same brightness (no one knows why they all should have the same brightness, but that's a small detail (It is useful to point out that supernovae all have relatively the same brightness because it requires a certain amount of mass to make a star go supernova. It is the distance to them that changes the apparent brightness). Because they all have the same brightness, they are very good for measuring distances, and are called standard candles because of this and Doppler shift must be taken into account given the large distances. The thing that has everyone confused is that it seems that the rate of expansion of the universe is increasing. This suggests that there is some sort of dark energy in space. As space expands, this energy comes out and makes space expand some more. Right now, one of the major research efforts is to get data on fainter supernova in order to have more points on the expansion curve line. The goal is to hopefully get a more exact idea of how the universe is expanding so that we have some idea of what is causing the acceleration.
This is still confusing people. One of the problems is that we have some pretty strong arguments as to what dark matter isn't. It's probably not regular matter made of protons and neutrons. The reason why, is that if it were ordinary matter, then when you calculate you get too little deuterium in the universe. Another problem is that the "clumpiness" of the galaxies and the cosmic background radiation just doesn't look right. If you go through the properties of dark matter, it turns out that it doesn't correspond to any known particle.
Some other theories explaining dark matter deal directly with our way of thinking about gravity. There is speculation that there is another large-scale force that is keeping our universe together. Another possible explanation is to think of space as a gas-and-space solid. If you place two objects apart from each other then pressurize the area the two objects will be forced towards each other. This is reversing our current ideas of gravity from an object having a pull on other object, to an object being pushed from all directions. (An object alone has no movement, but two objects create an uneven pressure pushing the objects together.)
An estimated 23% of the matter in the universe is dark matter. Ordinary matter only makes up 4% of the universe. The remaining 73% is an even more mysterious, repulsive "dark vacuum energy".
The most popular theory right now is that the repulsive force is actually a property of space itself: it is caused by waves of energy, created by particles and anti-particles popping into existence and then annihilating each other. Early in the universe's life, when there wasn't much space, the effect was small compared to gravity. But as the galaxies moved apart, the effect became greater.
ReionizationThe cosmic background radiation was formed when protons and electrons combined to form atoms. The trouble is that we know that the matter between galaxies today is ionized (i.e. it's separate protons and electrons) with clumps of hydrogen atoms. We know this because when we look at all but the most distant galaxies, we don't see the spectra lines of hydrogen. So at some point the hydrogen in the universe reionized. The notion was that starlight caused the hydrogen in the universe to reionize, but the latest observations seem to indicate that this reionization occured before the first stars were there.
Galaxy Formation
The idea is that galaxies started from tiny fluctuations in density that formed after the big bang. By assuming that the universe consists mainly of cold dark matter, you can almost get the clumpliness that you see with the current galaxies. But there are still puzzles. There is an annoying lack of tiny galaxies, and the rotation curve that cold dark matter predicts, isn't quite the one that we see.
Before the Big BangNow to get really speculative, there have been some papers written recently that try to figure out what happened before the Big Bang. One of the strange ideas is that the universe is merely one plane in a multidimensional space, and that what happened was that two membranes in a multidimensional space collided causing a massive expansion in three of the dimensions. This is all really speculative, but the weird thing is that it isn't totally disconnected from observation. The idea is that you can use this model to predict the initial expansion of the universe, and this might have some effects on the ripples that you see in the cosmic microwave background.
With all of these puzzles, its not clear what is going to happen next. There is a lot of data coming in, and it may be that with new data, it will be possible to make our models of the universe work with minor tweaks here and there, and we can go on in the mode of what Kuhn calls "normal science." It's also possible that one day there will be some observation which is like Galileo seeing the phases of Venus. Some observation that makes absolutely no sense in the current paradigm of things, and this will force people to fundamentally change how we view the universe.
From : Wikipedia
