Dark Matter - When Gravity Speaks

Everything we see is almost irrelevant in the grand scheme of things.

Look at the sky. Everything you may see - even if equipped with an imaginary hyper powerful telescope that can see every star, planet and galaxy in the cosmos - makes up less than 5% of the mass of the Universe. This tiny 5% is what we call “normal” matter. It interacts with Electromagnetic Radiation (anything from radio and microwaves to visible light and gamma rays), resulting in it absorbing, reflecting or emitting light. The remaining 95% of the Universe however, does not interact with electromagnetic radiation at all, making it invisible. Undetectable. So scientists called it Dark Matter. We don’t know what it is, how it came to be or how to capture it. We just know it is out there. Dark matter doesn’t shine, reflect, or absorb light - but it has one unmistakable signature: gravity. And across the universe, we’ve seen its invisible hand at work. Like footprints left behind in snow, it leaves traces, patterns in light and motion, that reveal where it’s been, and that it’s there.

There are two main phenomena which have inferred the existence of dark matter:

how stars spin in galaxies and how light bends after galaxy collisions.

The first puzzle appeared when astronomers looked at how stars spin in galaxies and realised they were defying gravity.

Imagine you’re swinging a ball on a string around in a circle. The ball wants to fly off in a straight line, but the string keeps pulling it toward the center. That pull keeps it moving in a circle. Now imagine the ball is a galaxy, and the tension in the string is the force due to gravity, due to the huge mass of the galaxy. The star wants to move in a straight line through space but gravity from the galaxy pulls it inward, so it keeps going around in a circular path. That pulling force toward the center is called centripetal force. In space, this is the gravitational force of massive objects.

The speed at which a star is moving around a galaxy, so how many revolutions it completes in a given time, depends on two things. The mass of the galaxy pulling it inwards and how far it is from the centre of the galaxy. The heavier the galaxy, the stronger the pull and the faster the star moves, but the further away the star is, the weaker the pull so the slower the star moves. Astronomers used this logic to calculate how fast stars should be moving at different distances from the center of a galaxy—and they plotted this on a theoretical graph. But when they actually measured the speeds of those stars, the results didn’t match.

The stars on the outer edges of galaxies weren’t moving slowly at all - they were spinning around far too fast, as if held by some hidden force. The visible matter we know simply wasn’t enough to explain it. Something hidden and unseen was holding these galaxies together. Something massive. That was the first observation to infer the existence of dark matter.

The second clue we have is rooted in light itself, and how the universe bends it. To picture how this works, imagine space like a giant trampoline. Place something heavy, like a bowling ball, on the surface and it creates a dip. Now roll a marble across the trampoline; it curves around the dip. That’s what happens when something massive bends space. Massive objects like galaxies warp the fabric of space, and since light passes through space, it bends too. Using very powerful telescopes, we can detect this bending of light, also known as gravitational lensing.

Using lensing, astronomers can do more than observe light. They can map where the mass truly is, including the kind we can’t see. This becomes especially powerful on the biggest scale; when galaxies crash.

As far as we know, galaxies are made of stars as well as gas and dust filling the galaxy’s seemingly “empty” space. When two galaxies collide, as stars are extremely spaced out, they rarely hit each other and pass through the collision mostly unaffected. But gas and dust are different. Since they are everywhere in each galaxy, their particles interact and crash continuously. As a result of these collisions, the particles lose momentum and their kinetic energy is converted into thermal energy so the particles slow down, clump together and get really hot; millions of degrees hot.

We all know that when something gets hot, it glows. When it’s just a little hot, like coils in a toaster, it glows red. When it gets hotter, like a candle, it glows white. But when it gets millions of degrees hot - like the gas in colliding galaxies - it gives off energy in the form of X-rays, that’s too powerful for our eyes to see, but detectable by space telescopes. So we see a glowing cloud of hot gas sitting right at the site of the collision. So most of the mass of the galaxy, except for that accounted for by the stars, should still be close to the region of the collision.

But this is where things get weird. Gravitational lensing showed that most of the mass was not where the hot gas was, it had moved ahead with the stars, completely unaffected by the crash, causing very noticeable bending of light away from the site of collision. Bending caused by heavy matter that doesn’t interact with light or with other particles, it doesn’t collide or slow down. It simply passes through. This was the second major piece of evidence for dark matter.

It’s easy to think of dark matter as just another cosmic mystery. But by studying dark matter, we aren’t trying to explain another mystery of the universe, we’re trying to complete the universe itself.

To understand where the universe is heading and how it will end, scientists need to answer a surprisingly simple question: How much stuff is actually in it?

This is called the average mass-energy density. In simple terms, it’s like asking if you took everything in the universe - all the stars, planets, gas, dark matter, and even energy - and spread it out evenly across space, how heavy would each part be. This matters because the universe is expanding; space itself is stretching outward. Whether it keeps expands forever or one day collapses in on itself depends on the delicate balance between its outward expansion and the amount of mass and energy pullung it inwards through gravity.

Think of the universe like a ball tossed into the air. The strength of the throw represents the expansion, and the weight of the ball is the universe’s mass-energy density. If the ball is light and fast enough, it keeps flying upward. If it’s heavy enough, gravity will slow it down and eventually pull it back.

And that’s where dark matter comes in. Without it, the universe doesn’t weigh enough. The gravity from normal matter is not enough to slow it down, let alone collapse it. But with dark matter, the scales tip. Suddenly, there’s enough extra mass to change the story - possibly even the ending.

Hence, while we may not be able to see dark matter, its fingerprints are all around us - in spinning galaxies, bent light, and the hidden weight of the cosmos itself. We feel its pull in every corner of space. It is the invisible architect of the universe, quietly shaping everything we know. And until we understand and uncover what it is, the ultimate fate of the universe will remain in the shadows.

Laura More