There are many inexplicable phenomenons happening in our universe that cannot be explained with the help of known visible matter. For example, universe is expanding at an accelerating rate, the galaxies in our universe are rotating with such high speeds that the gravity generated by their observable matter could not possibly hold them together, consequently, they should have torn themselves apart long ago, but this did not happen! So what is holding them all together ? Moreover, the galaxies don’t rotate by the same physics as a spinning plate. The stars at a galaxy’s edge rotate faster than expected. So what is the reason behind such a bizarre rotation pattern followed by these galaxies ? Now, this is where the terms dark matter & dark energy comes into existence!
Jacobus Kapteyn, a Dutch astronomer was the first one to suggest the existence of dark matter in 1922. Dark Matter is basically a hypothetical form of matter that probably accounts for approximately 85%of the matter in the observable Universe. According to the Standard lambda-CDM model of cosmology, the total mass energy of the universe contains 5% ordinary matter and energy,27% dark matter and 68% of an unknown form of energy known as dark energy.Thus, dark matter constitutes 85% of total mass, while dark energy plus dark matter constitute about 95% of total mass–energy content.
The majority of dark matter is thought to be non-baryonic in nature, which is possibly composed of some as-yet undiscovered sub atomic particles .These unknown particles are expected to be probably some weakly-interacting massive particles (WIMPs), or gravitationally-interacting massive particles (GIMPs). Other potential candidates include dim brown dwarfs, white dwarfs and neutrino stars. The WIMPs are expected to have ten to a hundred times the mass of a proton, but their weak interactions with “normal” matter makes it difficult to detect them! Dark matter has also been classified into hot and cold categories, depending upon the velocities of their particles which further determine their thermodynamic properties. The candidates for hot dark matter travel with relativistic velocities like neutrinos. However, the ones moving slowly like hypothetical axions, WIMPs etc come under the category of cold dark matter.
Unlike normal matter, dark matter does not interact with the electromagnetic force, hence, it does not absorb, reflect or emit light, making it extremely hard to spot. This is the reason why it had been named ‘dark‘. The only inference of its existence has been drawn from its gravitational effects on the visible matter. Several ultra sensitive instruments have been constructed to detect these bizzare sub atomic particles. These include vats of liquid xenon stored miles underground, and telescopes looking for dark matter particles decaying into things we can see and measure, like gamma rays. It also includes the Large Hadron Collider, one of the most expensive science experiments ever built.
Sadly, as these particles are highly non-interacting, so, even with these advanced instruments, we haven’t been able to find anything in favour of their presence till date.We haven’t found convincing evidence that they exist, except for the persistent evidence we can’t ignore the fact that the universe is heavier than what we can see. This fact is the sole motivation behind scientists all over the world putting forward various theories, day by day, in this context. Consequently,a lot of advancements are taking place in the hope to discover the unknown one fine day! Hence, the hunt continues…
You are entitled to say, if you are so smart, why don’t you tell me what that dark matter is? And, I’ll have to confess that I don’t know ! –Jim Pebbles ( A cosmologist making major contributions to the field of dark matter since 1970)