Our scientific understanding of the universe has had more than a few upheavals over the past century, beginning with revolutionary advances in theories of relativity and quantum physics. However, two puzzles continue to defy easy explanation: why regions around the universe seem to have more mass than our standard models of matter account for, and why the universe's rate of expansion is continuing to accelerate, when both standard models and common sense tells us that the force of gravity should eventually slow down the expansion and draw everything back together. Two new forces have been put forward in recent years to account for these baffling observations: so-called dark matter and dark energy.
It should be noted that, in both cases, referring to these phenomena as "dark" has nothing to do with the fact that they are black, or even that they emit no visual light per se. Rather, dark matter and dark energy are "dark" because we have no way of directly observing them through any means; at least for the moment, they exist primarily as theoretical concepts and as inferences from the empirical record.
- About Dark Matter -
In theory, as with all systems involving orbital dynamics, one can examine a galaxy in detail and estimate the rate of rotation of its outlying regions of stars and gas by understanding the size of the galaxy and the total mass of those regions of space (including the stars, gas, and dust present). However, once the modern model of galaxies had been put forward and technology became available to make these measurements, scientists quickly realized that there was a problem: the model didn't work. In the 1930s, Fritz Zwicky reported that movement of galaxies within the Coma galaxy cluster implied a total mass that was orders of magnitude greater than the estimates of the total physical matter present. In short, if the galaxies in question were as light as they were believed to be, they should have orbited each other relatively slowly; instead, they were orbiting very quickly. Since we are very confident in our ability to predict the force of gravity in a given situation, this prompted a search for explanations for the so-called galactic "missing mass."
Further studies of gravitational lensing and of the cosmic microwave background - a faint glow which pervades the entire universe and is believed to be a remnant of an earlier stage in its history, just after the Big Bang - have tended to support the theory that there is something other than normal physical matter out there, which has mass of its own but which we cannot directly observe through visual light or existing technology. Some physicists now believe that our own Milky Way galaxy contains several times as much dark matter as it does normal matter, and that there are other, difficult-to-see galaxies which are composed almost entirely of dark matter.
- About Dark Energy -
Dark energy sounds similar, but is actually a very different phenomenon which was devised to resolve a different theoretical problem. Beginning in the late 1980s, studies of white dwarf supernovae, and subsequently analysis of gravitational lensing and the cosmic microwave background, established that the universe is not only still expanding (which was understood by all advocates of the Big Bang theory), but that the rate of expansion is still accelerating. This came as something of a surprise: given what we know about gravity, the standard assumption was that the rate of expansion was decelerating (even if only very slightly so), and would eventually stop entirely. At that point, the universe would contract inwards, until, trillions of years from now, all matter would collide back together at a single point. This might give rise to another Big Bang: in other words, it implied that the universe was cyclically contracting and expanding, and we were simply living in one of potentially an infinite number of those cycles.
If the universe's rate of expansion was accelerating, however, this meant that for some reason the force of gravity had lost out to some other force which was pushing things apart faster than gravity could pull them together. The name "dark energy" was put forward to account for this mysterious phenomenon. The nature of dark energy defies explanation, although the best guess today is that all space, even the near-total vacuum of the intergalactic medium, has low-density energy. When the universe expands, the quantity of matter and dark matter becomes more dispersed: that is, less dense. However, when more space is "created" through universal expansion, more dark energy is created along with it.
If dark energy theorists are correct, our universe's history and destiny are more ominous than we suspected. For the first few billion years, matter and dark matter were dense enough that gravity won out and the universe's rate of expansion actually would have decelerated. However, at a certain point, space became so big - and dark energy so comparatively influential - that gravity started losing the battle, and the rate of expansion began to accelerate. At this point, that acceleration is essentially uncontrollable: barring the intervention of yet another force of which we are quite unaware, the universe's acceleration will continue indefinitely.
Indefinitely, but not infinitely. As expansion continues, the galaxies will fly apart from one another, so that in the very, very distant future, our descendants will look up into the night sky and it will be nearly empty. Alone, our galaxy supercluster will eventually run out of fuel and grow dark and dead. The most pessimistic projections even argue that this expansion will continue until it stretches apart the galaxies, and then the stars and planets within the galaxies, and then eventually destroys atoms themselves - a scenario known as the Big Rip.
Scientists now believe that the universe is made up of a little less than three-quarters dark energy, about one-quarter dark matter, and the rest normal matter.