Gravity allowed scientists to discover dark matter. In 1933, a Swiss astronomer, Fritz Zwicky tried to measure the total mass of a galaxy cluster by summing the mass of each individual visible galaxy in the cluster. He found out that their total mass was not enough to create the observed gravity that holds the galaxies together to form a cluster. With just the gravity created by all of their visible matter, the galaxies would not cluster easily, if at all. Thus, Zwicky concluded that there must be something invisible, inside and around the galaxies. This matter adds the extra mass to create that gravity, strong enough to form a galaxy cluster. Zwicky called this unseen mass, dark matter.
More evidence for dark matter has emerged over time. Photographs of galaxies showed that most of their light, i.e. most of their stars, were concentrated near the center of the galaxies. Therefore, most of the mass of a galaxy must be concentrated in its center, meaning that gravity is stronger at the center of a galaxy than in the outskirt. Because of this, it is expected that the stars near the center of the galaxy would move faster than those farther away. However, the measurements indicated that the orbital speed of stars was the same everywhere, regardless of their distance from the center. The conclusion is that there must be invisible matter that spreads throughout a galaxy, such that stars far away from the center will feel the gravitational pull of not only the central material, but all the other matter between them. The extra force of gravity from dark matter can cause them to speed up roughly to the same speed of the stars near the center.
The existence of Dark Matter is evident through an optical illusion, called gravitational lensing, seen as a result of light being bent by the gravity generated by dark matter. When viewing distant galaxies with telescopes, astronomers observed strange rings and arcs of light coming from a distant source, despite no observation of any mass. Light normally travels straight in space. The presence of mass creates gravity. Einstein discovered that gravity is the same as curvature of space, which causes light to travel along this curvature, instead of going in a straight line, i.e. light is bent. Therefore, the mass acts as a lens, called a gravitational lens, bending any light passing by. This indicates the presence of invisible mass that is creating the gravity and the phenomenon. Hence, dark matter can also be indirectly observed through gravitational lensing.
Although it’s been discovered for some time, it’s still unknown what dark matter really is. Scientists have proposed various ideas and designed experiments to test those ideas. However, the quest to define dark matter has, thus far,mainly been a process of elimination. Experiments have ruled out potential candidates, such as black holes, brown dwarfs, neutron stars, which are invisible or difficult to detect, but are not numerous enough to account for the additional mass in the Universe.
A current leading hypothesis is that dark matter is made of something different from the normal matter because it is invisible and does not interact with other normal matter that we are familiar with. Scientists think dark matter might be made of particles predicted by theory (exotic particles), however, they have been unable to find these particles yet. Scientists have also been trying to generate dark matter particles in large underground experiments where avoid contamination by other “normal” particles Can be avoided. In the absence of discovery of dark matter particles, some researchers suggest that the effects of dark matter could be explained by fundamentally modifying the theory of gravity. This idea proposes that at the scale of the Universe, gravity may act differently than on Earth, creating the effect that scientists interpret as dark matter.
Something even more mysterious, called dark energy, makes up 70% of the Universe, besides normal matter and dark matter. Normal and dark matter generate gravity which holds things together. Dark energy is completely opposite to gravity; it causes things to be pushed apart. To understand dark energy, think about what happens when you toss a ball up into the air. It goes up and gradually slows down due to the pull of gravity. Eventually, the ball stops in mid-air and falls back to the ground. Now imagine a ball, once tossed up, keeps flying up further and further, faster and faster, instead of being attracted back down to the ground. This event seems impossible to happen, but this is the property of dark energy.
In the 1990s, dark energy was discovered as astronomers observed such a peculiar effect in distant galaxies within the Universe. Scientists know that Universe has continually expanded, with galaxies moving away from each other, since its formation at the Big Bang. They also observed that the speed of expansion has increased, which is unexpected, because like the tossed ball, the expansion was expected to slow down as gravity pulled on all of the galaxies. Scientists had suspected that the Universe would stop expanding and finally collapse, if gravity won and halted the expansion. Scientists concluded that the accelerated expansion cannot be caused by dark matter and normal matter, which generate gravity for the Universe, but must be by some form of mysterious energy that acts as an force opposite to gravity. As it is invisible, they called it dark energy. Remember that the Universe has always been expanding; dark energy acts to accelerate this expansion. By measuring how fast the Universe is expanding over time, which is a combined effect of dark matter pulling galaxies together and dark energy pushing them apart, astronomers can determine the proportion between dark energy and dark matter.
Scientists have many theories, but no concrete answer yet, for the nature of dark energy. The different theories for dark energy differ in the way they predict the expansion of the Universe with time. Scientists are trying to take more accurate measurements of the acceleration of the Universe, to see which theory best explains dark energy and hope to understand what Dark Energy is. One other possibility suggested by some scientists is that maybe Einstein’s theory of gravity is flawed and there is no dark energy after all. However, no one has been able to improve on Einstein’s theory such that it can explain the effects of dark energy and also fit other phenomena of gravity. Dark energy remains one of the greatest mysteries of the Universe. If one day we understand dark energy, it will change what we already know and the way we think about the Universe.