Universe comprises mostly of blank space called vacuum, which has nothing.
the stars are mostly balls of helium and hydrogen, with some of them made only of neutrons, thus giving them a name of neutron stars. Planets like our Earth, consists of many elements. And we don't enen know what hides in the vastness of the space that is spread all around us.
But the entire universe is mostly comprised of hydrogen and helium upto 98%.
with Hydrogen almost upto 90 %