Answer:

the name of the biggest thing of the universe is dark matter which is everywhere like a chain in our universe

and the reason behind the expansion of universe is the anti gravitational pull made by the dark matter which tends the universe to expand

Explanation:

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The expansion of universe is taking place from the beginning of it's birth.

Our universe is expanding due to the property of inertia, and the dark energy or decaying dark matter.

The biggest thing in the universe is yet not proved yet , but the biggest thing found is actually a supercluster called Hercules- corona Borealis .