Dark Matter: 85% of Everything
Look at your hands. You probably already know, from primary school, that they’re really just a bunch of atoms, all stacked together neatly. Everything around us, everything that exists, is also made up of these microscopic, subatomic particles –– that is –– everything we can see. Scientists have long since found out that what we can see isn’t even a small portion of everything that actually exists.
This is (a representation of ) the Milky Way:
It weighs about the same as 1.5 trillion Suns put together. What if I told you that everything visible, all the stars and gas and planets, only made up about 12% of that mass?
That’s right, a whopping 84% of all the mass in the Milky Way (and in the whole Observable Universe) is stuff that we can’t just not see, it’s stuff that we can’t measure, can’t interact with and can’t even comprehend.
It’s dark matter.
Don’t worry, things get even weirder. In the 1960s, astronomer Vera Rubin realised that since the centre of the galaxy is denser, the rotation of the spiral should be fastest at the centre and slow down as we get farther away from it. That’s just how gravity works. But what she found was shocking –– the speed of rotation remained constant throughout the entire spiral. The only way to explain this bizarre finding without changing the idea of gravity entirely was to conclude that there was something else in the galaxy: something heavier, and yes, something completely invisible.
So, scientists decided to play a game of “what can it not be?”. They slowly crossed everything off the list –– cold gas, dead stars, galactic dust –– and turned up empty. This meant that whatever dark matter was, it wasn’t made up of the same subatomic particles as normal atoms; no protons, neutrons or electrons.
This led to the theory that there was perhaps another kind of subatomic particle we hadn’t discovered yet. Scientists Peccei and Quinn called these hypothetical particles ‘axions’ –– a whole new elementary particle with a very low mass. This mass would add up, however, and cause the strange gravitational effects we just discussed.
This isn’t the only theory, however.
Since dark matter and dark energy are so elusive, there’s no shortage of possible explanations. One theory suggests that dark matter is just a property of the fabric of space-time; that since space expands at an accelerating rate, there must be some sort of energy driving that expansion. This is because the whole phenomenon of space growing and expanding in on itself is bizarre: space should actually be shrinking due to gravity. But some force that doesn’t act like any other force we’ve seen so far is causing the opposite to happen.
Look at this beautiful image taken by the Hubble Telescope:
In the centre is a luminous red galaxy, enveloped by what looks like a blue ring. It’s just an illusion, however. The blue ring is actually another galaxy, far, far away. The light coming from that galaxy to the Earth is interrupted by the huge gravitational effect of the red galaxy in the front, thereby bending and forming a distorted image, just like in a spherical lens.
But remember what we just found out? The visible stuff in the foreground galaxy doesn’t have nearly enough mass to cause such a large gravitational effect. It’s the dark matter in these galaxies that causes the light to bend.
So yes, we have more than enough evidence of dark matter and energy –– we just haven’t observed it directly yet. The idea that 85% of the Universe is made up of something that we cannot touch, perceive or understand is incomprehensible yet true. It really makes you think –– what else do we not know just yet? Will we ever know all of what there is to know about the Universe, or will we just continue to live our lives in total oblivion –– in the dark?
