Think about all of the stuff in the universe, from the particle of dust on your screen, to the billions of planets and galaxies you see in the night sky. Everything you could possibly imagine- all the observable matter in the universe- is only 5% of what is really out there. Everything else is dark. Science development led us to more and more sophisticated and accurate technology to investigate the cosmos and little by little astronomers and physicists have learned a lot about the universe, discovering stars, stardust, planets, satellites, asteroids, galaxies, nebulae, clusters of galaxies, and so on...nevertheless, starting from the last century, observations suggest that there is far more to discover. As a matter of fact, all that exists in the universe seems to be much more than what we can detect. The mass we can see and measure is only a minimum fraction of the whole universe because the larger amount of matte is invisible and undetectable by our instruments. It is like an immense halo that permeates the cosmos without emitting, absorbing, and reflecting nor light neither electromagnetic waves. This is the dark side of the universe, the dark matter.




What Is Dark Matter?
What Is Dark Matter?


 

 A property of most normal matter is that it interacts with electromagnetic radiation or light. This means that when light reaches an object, it is typically absorbed or reflected. This interaction allows us to detect this matter. so, if we cannot see dark matter, what makes us believe it is there? 

It all has to do with another property of matter, the effect it has on other matter in the universe. We call this phenomenon gravity and experience it as a pull that the mass of the sun exerts on the planets around it, keeping the earth in orbit. When any two objects orbit each other, this gravitational pull is what keeps them from flying apart. The faster two objects are orbiting each other, the more mass we expect them to have. In 1933, astronomer Fritz Zwicky noted something extraordinary while studying a cluster of galaxies. He measured how fast the galaxies were moving around each other and used this to estimate their mass. When he compared his estimate to the observable mass of the galaxies, he found it was 400 times larger. He argued that there must be something missing or invisible mass and called it dark matter allowing them to orbit so fast without flying apart. This phenomenon has since been observed and verified by other scientists in many different galaxies including our own. But this speedy orbiting is not the only reason scientists believe dark matter exists. Another phenomenon related to mass is that it affects the way light moves through the universe. In a phenomenon called gravitational lensing, light bends around massive objects as it travels near them through space- the more massive the object, the greater the bending. By observation the light coming from distant stars, scientists have measured stronger than expected bending around galaxies, indicating that the galaxies have more mass than we can see.

 

So what it really is ? since we cannot observe it, scientists are not really sure. But there are a number of theories as to what it might be. One theory is that dark matter is made of massive compact halo objects or MACHO. MACHOs are very large collections of normal matter that do not typically interact with light and thus are not visible. Physicists observe these objects through phenomena related to their gravity. Black holes are an example of such objects, and can only be detected by the way light and matter collect around them. Unfortunately, black holes and other MACHOs seem to exist only in distant pockets in the universe, whereas observations show that dark matter is spread out more evenly in space, so many scientists reject this theory. Instead, many physicists believe that undiscovered particles are what constitutes dark matter. The theory requires the existence of a special group of particles called WIMPs- weakly interacting massive paricles. Scientists expect them to have certain properties, mass, and inability to interact with light, which fit well in the description of dark matter, but since we cannot observe them, we cannot be certain that they are there.

 

 Physicists are trying to detect dark matter through experiments on earth and observations of the universe, but even with modern advancements in science, we have not been able to pinpoint exactly what dark matter is. The search for the missing matter remains one of the biggest unsolved problems in physics, and it waits for someone to solve it.