Stars orbit around the galactic centre (usually a blackhole) just as planets around the Sun.
Now a general law which talks about orbital speeds of planets is Kepler’s law.
According to Kepler, owing to gravity, closer the planet from the sun faster it moves farther the planet slower it moves.
This should be true for orbital speeds of stars going around in a galaxy too. However observations show that all stars, closer or farther, have same orbital speeds.
Something should be slowing down the stars in the orbits. If you calculate the combined gravitational effect of all the luminous objects like stars and galactic centre of the galaxy, it is not enough to hold the stars in the galaxies.
The missing matter that is exerting huge gravitational attraction and slowing down the stars in a galaxy is called dark matter.
In addition we have seen how galaxies are clumped in a local group held together by gravitational attraction. If you measure the speeds of galaxies (using redshift) they are so high that galaxies should have raced out of the confines of a cluster. For them to clump together something should be holding them, pulling them inward. This is also because of dark matter.
Composition of the universe: Out of all the matter the universe is made of 63% constitutes dark matter.
About 27% of everything in the universe is made of Dark Matter.
What is this dark matter made up of?
There are many familiar astronomical objects that are invisible, such as blackholes, neutron stars and black dwarfs.
But observations showed that they could form only a small fraction of the dark matter in the Universe.
As a result, hypothetical particles have been considered to account for the dark matter, such as Weakly Interacting Massive Particles—WIMPs.
WIMPS are hypothetical particles that are assumed to make up for the dark matter.
LUX-ZEPLIN is the detector that is trying to find WIMPs in other words dark matter.
What are WIMPs?: In short, nobody knows. What is dark matter made of? nobody knows. That’s why it’s called ‘dark’.
Dark matter and gravitational lensing
You have seen in Einstein’s General Theory of Relativity how matter affects spacetime. In other words matter warps spacetime and this you call gravity. That’s why gravity bends of light.
Now suppose you have a far off bright source like a star. Imagine there is a galaxy between that star and the observer on earth.
Then the light from the star can be bent by the strong gravitational field of the galaxy coming to a focus at our telescope. Then we see 2 images of the star as though they are coming from 2 different sources as image 1 and image 2 as shown in the figure. This is called gravitational lensing.
Note: An image of gravitational lensing captured by Hubble Space Telescope is given below
Now, replace the galaxy by the invisible dark matter. It will act as a lens, just like the galaxy, although it is ‘dark’.
In other words dark matter also produce the effect of gravitational lensing.
Einstein Ring
Gravitational lensing of objects like galaxies or cluster of galaxies due to massive object(another galaxy, supernova, dark matter) in between the observer and the galaxy gives a ring-like appearance to the galaxy. This is called Einstein’s ring
Hubble space telescope and James Webb space telescope have captured pictures of near-perfect Einstein’s ring.
