Space.com posted an article on June 15, 2017, which is a good summary of this topic — “What is dark matter?”
Roughly 80 percent of the mass of the universe is made up of material that scientists cannot directly observe. Known as dark matter, this bizarre ingredient does not emit light or energy. So why do scientists think it dominates?
Most scientists think that dark matter is composed of non-baryonic matter.
If scientists can’t see dark matter, how do they know it exists?
The article summarizes some experiments trying to detect this stuff.
Another useful source for background on this topic is Dan Hooper’s book.
Hooper, Dan (2009-01-09). Dark Cosmos:In Search of Our Universe’s Missing Mass and Energy. HarperCollins. Kindle Edition. [Copyright 2006, prior to the LHC going online and Planck satellite]
Dark matter is a hypothetical type of matter distinct from baryonic matter (ordinary matter such as protons and neutrons), neutrinos and dark energy. The existence of dark matter would explain a number of otherwise puzzling astronomical observations. The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum. Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter,gravitational lensing, its influence on the universe’s large-scale structure, on galaxies, and its effects in the cosmic microwave background.
The apparent expansion of the universe boggles the mind:
… studies of distant supernovae revealed that the universe today is expanding faster than it was in the past, not slower, indicating that the expansion is accelerating.