In 2018, astronomers discovered a galaxy with almost zero dark matter. This was a groundbreaking discovery as scientists always considered dark matter to be a crucial component of galaxies. This galaxy named NGC1052-DF2, is 65 million light-years away from us in the constellation Cetus. It is of the same size as Milky Way but has one star per 200 found in Milky Way. Because of lack of stars and absence of dark matter, it has a very low mass.
The main evidence of dark matter is the calculations which show that galaxies would have flown away instead of rotating if they did not contain matter which is not visible otherwise. As dark matter has not been observed directly, it must have a bare minimum interaction with ordinary baryonic matter, except only through gravity. In the standard model of cosmology, dark matter consists of about 85% of total mass while dark matter and dark energy combined accounts for 95% of total mass energy content.
Now again, researchers have found one more galaxy without any dark matter. This one is named NGC1052-DF4. The research team at Yale University which discovered DF2 was skeptical about its findings, but now the discovery of DF4 clears the confusion and it proves that galaxies do exist devoid of any dark matter. This galaxy like the previous one is also an ultra diffuse galaxy, large, spread out and can be faintly observed. These galaxies have very less stars thus making them very difficult to observe.
The astronomers tracked orbital motion of multiple dense clusters of stars which are called globular clusters using a device called Low Resolution Imaging Spectrometer. This velocity was found to be consistent with the one calculated using the mass of the normal matter of the galaxy. Scientists are hoping such galaxies can be found in more numbers and more research work can be carried on how they were formed.
Scientists also conducted more research work to confirm the past results for DF2. They used the Keck Cosmic Web Imager and tracked the orbital motion of 10 globular clusters. The results were consistent with a lack of dark matter.
But it has not been found yet that why these galaxies do not have dark matter. This is still unsolved. It might be possible that they had dark matter present but then it was lost. There can be multiple hypotheses to this question and this can only be solved by finding more such galaxies. Researchers want more evidence before drawing any conclusion, so that they can finally solve the mystery of dark matter.