Many astronomers believe that we can only see about 5% of the matter in our universe. The rest is referred to as 'dark matter,' and while astronomers can't see it, they've been theorizing its existence and impact on the universe since the early 1900's.
Dark matter is used to help explain various inconstancies found in the universe, such as why hot galaxy clusters stay together, why stars remain bound to galaxies despite their rotation, and temperature differences observed in the CMB (cosmic microwave background radiation).
While dark matter hasn't been proven or disproven by any lone scientist yet, it remains one of the most viable modern theories. Then again, disproving its existence would throw a wrench in the whole ordeal and force scientists to start from square one when explaining these inconsistencies.