It has been suggested that 80% of all matter in the Universe cannot be detected. It has been called Dark Matter, an all encompassing term that covers a lot of different theories about what this matter might actually be. There are also others who claim that Dark Mater does not exist and that there are alternative explanations for why elements in the Universe behave as they do.
Astronomers found that the mass of large astronomical objects calculated based on their impact on gravity was substantially larger than the mass calculated from what the object might have contained – for example the total mass of gas in a star.
Jan Oort in 1932 suggested Dark Matter as the source of the spinning speeds of stars. Then in 1933, Fritz Zwicky said that dark matter might explain “missing mass” in the spinning speeds of galaxies in clusters. Later, many other observations have suggested that there is dark matter in the universe:
- Spinning speeds of galaxies,
- Gravitational lensing of background objects
- Temperature distribution of hot gas in galaxies and clusters of galaxies
Roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest is less than 5% of the universe. So everything we see and generally think about as being the contents of the Universe is actually not the norm.
As no instrument has yet detected the dark matter, it must be that this material is not made of the same thing as the matter we see every day on Earth. If it were not for our understanding of gravity we would not be even thinking about it.
Some theorists have suggested that a modified versions of general relativity could explain the unusual behaviour of gravity. But no one has yet created a model that can sustain what we know about the Universe. If you change one thing about either Newtonian or Einsteinian physics, then you will have to change many other things.