Dark Matter The Matter We CanT See

The Ancient Greek’s Universe Theory

The ancient Greeks had a simple yet elegant idea about the universe. They believed that the universe was made up of four elements: earth, air, fire, and water. By combining these four basic elements in different ways, they thought they could produce all the wonderful diversity of the universe. For instance, earth and fire would give you things that are dry, while air and water would result in things that are wet. However, this theory had a significant problem. It didn’t predict anything that could be measured, and measurement is the basis of experimental science. Worse still, the theory was incorrect.

Leucippus’s Atom Theory

In the 5th century B.C., Leucippus of Miletus, a great scientist of the mind, proposed a more enduring scientific idea. He suggested that everything we see is made up of tiny, indivisible bits of stuff called atoms. This theory was simple, elegant, and unlike the earth, air, fire, and water theory, it was right. Centuries of scientific thought and experimentation have established that the real elements, such as hydrogen, carbon, and iron, can be broken down into atoms. In Leucippus’s theory, the atom is the smallest, indivisible bit of stuff that’s still recognizable as hydrogen, carbon, or iron. However, his idea was not entirely accurate as atoms are, in fact, divisible.

The Dark Universe

Leucippus’s atoms and the things they’re made of actually make up only about 5% of what we know to be there. Physicists know the rest of the universe, 95% of it, as the dark universe, made of dark matter and dark energy. We know this because when we look at things, we see them. All the stuff that’s made of atoms is visible. Light bounces off it, and we can see it. However, when scientists first measured the motion of groups of galaxies in the 1930’s and weighed the amount of matter they contained, they found that there’s not enough visible stuff in those groups to hold them together. This led to the discovery of dark matter.

Dark Matter and Dark Energy

The best evidence for dark matter today comes from measurements of something called the cosmic microwave background, the afterglow of the Big Bang. All of the evidence we have says that dark matter is there and it accounts for much of the stuff in those beautiful spiral galaxies that fill the heavens. Furthermore, scientists studying the outer reaches of the universe confirmed that not only is everything moving apart from everything else, as you would expect in a universe that began in a hot, dense big bang, but that the universe’s expansion also seems to be accelerating. This led to the discovery of dark energy.

The Composition of the Universe

Today’s best measurements allow us to work out just how much of the universe is dark. It looks as if dark energy makes up about 68% of the universe and dark matter about 27%, leaving just 5% for us and everything else we can actually see. So what’s the dark stuff made of? We don’t know, but there’s one theory, called supersymmetry, that could explain some of it. Supersymmetry, or SUSY for short, predicts a whole range of new particles, some of which could make up the dark matter.

The Universe’s Wonderful Diversity

There are theories that go back to that first great idea of the ancient Greeks, the idea that the universe must be simple. These theories predict that there is just a single element from which all the universe’s wonderful diversity stems, a vibrating string. The idea is that all the particles we know today are just different harmonics on the string. Unfortunately, string theories today are, as yet, untestable. But, with so much of the universe waiting to be explored, the stakes are high. Despite the vastness and complexity of the universe, we should marvel in the fact that, as far as we know, we are a member of the only species in the universe able even to begin to grasp its wonders, and we’re living at the right time to see our understanding explode.