Question

what do scientists know about the future of the universe?
what dark matter is made of
why the expansion of the universe is accelerating
that stars will eventually burn out causing the universe to become cold and dark
that galaxies spin much slower than they should based on visible matter

Explanation:

The first, second, and fourth options describe unsolved mysteries or observed phenomena that scientists are still researching to understand. The third option is a well-supported scientific prediction about the long-term future of the universe, based on our understanding of stellar life cycles and thermodynamics.

Answer:

that stars will eventually burn out causing the universe to become cold and dark