All the atoms in the world (including living beings, non living, all stars, galaxies, debris) constitute less than 5% of the total universe...

The universe is largely made up of Dark Energy and Dark Matter.

Even super sense neutron stars and even black holes are part of same 5% atoms. So what exactly is dark matter? What is dark about it? Do scientists know nothing about it?

In 1970s Scientists were studying spiral galaxies, and according to the known laws of physics, stars at the centre should move faster than stars at the edges of galaxy

But instead they found that stars were moving at almost the same rate as if under an unknown gravitational pull

Difference between measured and observed values was staggering. They tried different models to explain the anomaly. But they couldn't unless they assumed that there is unobservable matter whose gravitational pull is impacting the stars at the edge of galaxies. This is dark matter

Does it mean that dark matter is just notional? It's just a theory? Well even black holes are not directly observable. Neither are Higgs Bososns. But their existence was confirmed by some other observations. E.g. Dark matter causes gravitational lensing.

As per Einstein's theory, strong gravitational forces can bend light. So we do see light from far away objects bending due to massive gravitational pull of inbetween galaxies. This is called gravitational lensing

The amount of light bending that can be attributed to visible matter is again much lower than what's observed . Only when scientists account for dark matter inside galaxies the amount of light bending makes sense.

Ok, so we know that there is some unobservable matter in universe with extra gravitational pull than we thought. Should it not cause the universe to collapse under its own gravity?

Enter Dark Energy!!! The scientists know that the universe is expanding ever since Big Bang. But the rate at which it is expanding is more than we can explain by the standard model. In fact universe has picked up speed and expanding faster now.

When you throw something into the air, it eventually loses steam and earth's gravity eventually pulls it back. Big Bang created the initial force and sent everything flying around. But it should eventually lose momentum and slow down... Except that it has started accelerating!

So dark matter is what is pulling the universe together with its gravity, while dark energy is ripping it apart. They have been at odds with each other, like Yin and Yang.

According to the current Standard model of Physics, there are 4 primary forces in universe - gravity, electromagnetic force, weak nuclear and strong nuclear forces. They have been explained very well as an exchange of shortly lived particles.

This model already had trouble including gravitation to create a "grand unified theory of everything" (Hint: Stephen Hawking)

But Dark energy has broken it totally. We now think that it maybe the 5th fundamental force of nature.

Cosmology in general is a bright new field (about something ancient and futuristic ) . Our understanding is still far from complete. I don't even know if science can explain everything. But it doesn't cease to amaze me...

PS: I am not a physicist. Just a physics enthusiast.