Dark Matter and Dark Energy are as mysterious as they sound. To understand what dark matter is, you must first know what matter is.
Stars, galaxies, rock, planets, trees, water and even us(humans and other lifeforms) are made up of matter which is nothing but protons, neutrons, and electron forming atoms. Everything we see around us is matter.
But this matter(that we see around us and in the universe) only accounts for less than 5% of the known universe. Then what is the rest 95%? Is it not matter? The rest 95% is dark matter and energy, out of which about 25% is dark matter and about 70% is dark energy.
We have no clue what dark matter and dark energy is but we are pretty sure that they exist. So what do we know about them?
What is Dark Matter?
Dark Matter is the stuff that makes the galaxies and stars to exist(we can say so because it is the most abundant matter around, although we can't see it). When scientists calculated why the universe is structured the way it is, they found that there is just not enough normal matter to hold everything together. The gravity of the visible matter is not enough to form galaxies and stars. So there has to be something else inside and around galaxies which is causing them to exist. Something that does not emit or reflects light. Something that is dark. This gave birth to the term Dark Matter.
In the deep universe, wherever there is a high concentration of dark matter, light bends, which confused the scientists initially, but eventually, they realized that there is something which is not visible to us which is causing these optical illusions and interacting with gravity.
Dark Matter is scattered all around the universe and there are a few things which we can say for sure about the dark matter. First is that it interacts with gravity and second is that there is a lot of it.
What is Dark Energy?
Dark Energy is even more mysterious. We can't detect it, we can't measure it, we can't feel it but we do see and observe its effects very clearly.
In 1929, Edwin Hubble(the man after which the Hubble Telescope was named) examined that the wavelength of light emitted by distant galaxies shifts towards the red end of the electromagnetic spectrum as it travels through space. He found that more distant galaxies show more redshift than the closer galaxies.
So Hubble determined that it is because the universe is expanding itself. The redshift occurred because the wavelength of light was stretched as the universe expanded. Also, earlier it was thought that the expansion of the universe was decelerating and at some point in time in the future it might stop expanding and will start contracting. But more recent discoveries have shown that expansion of the universe is accelerating.
Space does not change its properties as it expands, there is just more of it created. Galaxies are tightly bound clusters of matter held together by gravity so we don't experience this expansion in our daily lives but where there is empty space in the universe, more of it is forming every second.
Coming to the accelerating expansion of the universe, scientists believe it is because of some repulsive force being exerted by the empty space, this force they call Dark Energy. So the dark energy seems to be some kind of energy intrinsic to the empty space. The energy that is stronger than anything that we know and that keeps getting stronger as time passes by. Empty space has more energy than everything else in the universe combined.
Theories Around Dark Energy
Scientists have suggested multiple ideas that what this dark energy might be.
One idea is that the dark energy is not a thing but just a property of space. Empty space is not nothing, it has its own energy, it can generate more space and its quite active.
Another idea was given by a well-known scientist Albert Einstein, that idea is of a cosmological constant, a force that counteracts the force of gravity. The only problem is that when scientists tried to calculate the amount of this energy, the results were so wrong and weird that it only added to the confusion.
Another idea is that empty space has actually consisted of some temporary virtual particles that spontaneously and consistently form from nothing and disappear into nothing again. The energy from these particles may be Dark Energy.
But it's still a work in progress. There are still a lot of questions to answer. Our theory about Dark Matter and Dark Energy is just a theory. So keep exploring and maybe one day you can give your own theory about Dark Matter and Dark Energy.
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