The more we observe about the workings of the universe, the more we reveal the extent of our ignorance. To understand the workings of the galaxies requires the belief in the existence of dark matter, something we can neither see nor, yet, detect.
In our physical models, the weight of all observable matter (stars, galaxies, etc.) accounts for just 5% of the universe, whereas an invisible, unknown matter that no tool has ever been able to detect appears to populate the universe on a massive scale. Yet, based on what we think we know, without the presence of dark matter galaxies would fly apart.
The filmmakers follow several physicists and cosmologists in their colossal experiments to capture a particle of dark matter, without success to date. And the scientists all recognize how little we truly understand about the cosmos.