They are looking for dark matter, obviously. It is still the most parsimonious explanation that there is some kind of gravitationally interacting stuff out the

They are looking for dark matter, obviously. It is still the most parsimonious explanation that there is some kind of gravitationally interacting stuff out there that we just cannot detect with our current measurement methods. It is the simplest hypothesis that can explain all of the available observations without adding too many epicycles to our otherwise well tested theories.