There are wee objects called dwarf galaxies which get their names from their small numbers of visible stars. The dwarf galaxies are expected to have high amounts of dark matter compared to other types of galaxies. But therein also lies the problem: in order to measure the amount of dark matter one has to see the stars, but in dwarf galaxies stars are sparse.
The scant few stars that we do see in dwarf galaxies give us the important indication that a huge mass underlies them. This is similar to the observation that seeing the tip of an iceberg indicates that a large mass underlies it.
In the December issue of Nature magazine, astronomers managed to measure the dark matter content inside of a dwarf galaxy called the Sculpter. The Sculpter is the poster child for this measurement because at only four times the distance to the Andromeda galaxy (M31), it is not so far away. Sculpter is also important because we have been making posters of this wee object for over 10 years using both the Hubble Space Telescope and GAIA satellite.
This long time baseline is sufficient to see how the positions of the stars in the poster from 10 years ago have changed compared to the positions of the stars today. The rate at which the stars change in position is related to how fast they move and, in turn, indicative also of the amount of dark matter that controls their motion.
At the end of the day, the higher the velocities of the stars, the greater the amount of the dark matter. A new public release of GAIA data is coming in April, 2018. These data will be used, in part, to strengthen this result that can take us one step further towards answering the big question in physics of what is dark matter?
