In my youth, I regarded the universe as an open book, printed in the language of physical equations, whereas now it seems to me as a text written in invisible ink, of which in our rare moments of grace we are able to decipher a small fragment. ~ Arthur Koestler
It's a great big dark universe out there. It's not just dark because there's nothing to scatter the light of the stars and brighten things up. It's dark because the vast majority of what makes up the universe is invisible. The concept of “dark matter” has been around for some time now, and more recently, physicists have determined that something known as “dark energy” is an even greater contributor to the stuff of the universe.
Scientists began figuring out that something was missing when they began to try to determine whether the universe was going to expand forever or contract back into a “Big Crunch”. To do this, they began adding up the mass of the visible universe. I think we can safely assume that there was statistics involved here. At any rate, as they began to look at the mass figures and compared them to other things, like the Hubble constant and the apparent gravitational effects on galaxies, it became clear that something was seriously awry.
It appeared that a great deal of our universe wasn't visible at all. Over the years, the figure for the amount of missing mass has been juggled around, but it would be fair to say that 80-95% of the “stuff” out there can't be seen. But it got spookier.
When Einstein was developing his theories, he found that his theory of General Relativity had a problem in that, without some sort of repulsive force, all the planets, stars, and galaxies would be clumping up. So he added a fudge factor that he called the “cosmological constant,” evidently figuring that if he gave it a grandiose name, people would overlook it's seemingly artificial nature. When Edwin Hubble discovered that galaxies were, in fact, flying apart from each other at high speed, Einstein gleefully removed the constant, thankfully eliminating what he called his “greatest mistake.”
Only it wasn't a mistake. For years, it had been assumed that the expansion of the universe was slowing down, as one would expect after an expansionary event like the Big Bang. Recent observations seem to indicate that the rate of expansion is actually increasing; in other words, there is some repulsive force at work. The cosmological constant is alive and well.
The cause of this repulsion was finally categorized as “dark energy.” Moreover, when looking at the missing mass problem, it appeared that there was as much as three times as much dark energy as there is dark matter. So let's summarize: Nine-tenths of the universe is made up of stuff we can't see, and we don't know what it is.
In scientific circles, this is called “a fine howdy-doo.”
But, those daring men and women with the science machines are making some progress. On one front, what may be the greatest science machine of all, the Hubble Space Telescope, has been used to attempt to map the distribution of dark matter. Even though you can't see dark matter, you can see its gravitational effects on visible stuff, like galaxies. The Hubble was used in concert with ground-based telescopes to develop a three-dimensional picture of these effects.
The map used observations of half a million galaxies. Since we know through statistical methods what the galaxies should look like, if their shapes are deformed in some manner, it would mean that something exerted gravitational influence on the light from those distant galaxies before it got to our instruments. The map only covers a small fraction of the sky, but it's an impressive piece of work nonetheless.
It also has some difficulties. Dark matter is normally associated with some sort of visible matter, most often as an invisible halo surrounding a galaxy. In some cases, the map shows dark matter with no associated visible material. It's not that there is some law that says dark matter can't be around with no visible stuff, but it's not expected. This could be due to some anomaly in some measurements, or it could indicate that there are problems with our models of the universe. At this point, it's too soon to start changing models, and the data that has been gathered will keep the cosmologists busy for some time.
On the dark energy front, a recent study of distant supernovae has shown that dark energy has been at work for at least 9 billion years. While this should have the cosmological constant crowd whooping it up, the data has one inconsistency. According to some current models, dark energy should have been an attraction force that far back in time. As matter density decreased, the force would have turned into a repulsive force. There was a point, therefore, where the dark energy forced flipped from attractive to repulsive. Unfortunately, the study doesn't indicated that to be the case.
The problem is that the data itself has a rather large margin of error, not unusual when looking at something this far away. About all that can be said for sure is that the data indicates the presence of dark energy in the young universe, but it doesn't necessarily depict its nature accurately.
Dark matter and dark energy are almost certainly part of our universe, but we are a very long way from understanding the properties of either one.
Well, no one said this was going to be easy.