The universe is mostly ghostly, according to scientists. It's a huge embarrassment for astrophysics that more than 96% of the stuff in the universe has them baffled. Invisible, intangible, and totally illusive, these dark things do not lend themselves to any kind of direct scrutiny.
Dark matter: Mathematical models say that the edges of galaxies should be flying off into deep space at the rate they are going. Confident that they had identified all ordinary matter (that which consists of molecules, atoms, and subatomic bits), physicists concluded that there must be some kind of invisible matter that adds enough gravity to keep the galaxies together, thus dark matter was invented. Nothing else is known about it. It is not to be found on the on the period table nor on the standard model of particles.
Dark Energy: Einstein, thinking a proper universe ought to remain the same size, was disconcerted when his relativity equations suggested the universe must expand or contract. So, he inserted a cosmological constant into the math to steady everything, an act that he later regretted as the biggest mistake of his career when Edwin Hubble proved that the universe was expanding. The world of physics assumed that the gravity of the universe would slow its outward expansion and were shocked in the 1990s when observational evidence proved just the opposite. Physicists concluded that there must be some kind of antigravity-like force at work and dubbed it dark energy.
So, here we are, inundated with darkness without the tools to qualify its nature, about a century after a famous Nobel laureate said. “Physics, as we know it, will be
over in six months.” (Max Born, 1928) Nowadays, most scientists maintain a modicum of humility in
the shadow of a growing mountain of unresolved mysteries.
What percentages of the universe's mass do dark energy and dark matter comprise respectively? Estimates have varied so widely over the early years of the 21st century that quoting sources would be unwise. However, the estimate for total dark stuff (matter plus energy) has remained steady around 95%.