What does Hinduism says about Parallel universe / Multiverse?

According to Hinduism Brahma was created by Lord Vishnu and assigned the task to create the universe. Brahma obliged and created the whole universe. One day of Brahma is a few Billion years and when it is night everything gets destroyed and re-created the next day.

How about multiverse where the science is talking about multiple universes out there?

Right, to answer this question we need to look into the stories on Lord Krishna's life.
Brahma totally forgot that Krishna is avatar of Vishnu and kidnapped all Gopas and their cattle for a year. During that time Krishna was there in every house as a Gopa and cattle in Vrindavan. Brahma returned after one earth year just to find that everything was as is and all the Gopas and cattles were still present though Brahma had kidnapped them.
Then Brahma saw millions of Brahma coming to salute Krishna and all the Vishnus going to Krishna, those Brahmas were assigned the role to create different universes and each and every Brahma was responsible for creating a unique universe.
Brahma realised his mistake and asked for forgiveness from Krishna.

So we have Lord Vishnus in Krishna and Krishna has everyone in him.
Brahmas and Vishnus are specific to a universe.

So this explains multiple universes out there.
Scientists have seen high pressure points near the border of the map of universe and reckons there might be other universes present.

Spirits explained scientifically

We all know E=MC² (Read as MC square)

Here M is mass, C is speed of light a constant.
So M = E/C² which means when the mass increases energy decreases and when mass decreases energy increases.

So for the mass to be negligible we need to be travelling to close to the speed of light.

Consider E = PE + KE
Where E is the total energy of the universe which is constant.
PE is the potential energy which is the energy when the body is at rest.
KE is the kinetic energy which is the energy when the body is moving.

So when the body moves close to the speed of light the PE becomes zero and all the energy is in the form of kinetic energy. Also when the body is at rest all the energy is in the form of potential energy.

SO when all the energy is in the form of kinentic energy and the speed is close to speed of light the mass become negligible and all the mass is converted to energy.

When someone dies and after the cremation the part of the mass is converted to small mass and part is escaped into the universe int he form of spirits.

So don't be afraid when you die. You will still be there in the form of energy ready to get converted to mass by rebirth.

Peace!!!