"he reminded us that less than five percent of the matter and energy in the Universe is understood"Always a good start to scientific investigation, "we have no idea what's going on, but let's use our current assumptions to infer a lot of stuff".
"Along the way, he provided a taste of the evidence for why we believe the things that we do. We know dark matter exists because galaxies don't fly apart, because we find gravity where there is no matter, and, most tellingly, our universe would be smooth and featureless in the absence of dark matter."
- Galaxies are far away, and light is slow. These galaxies might be flying apart, we wouldn't know for a long time.
- We "find gravity" by inference of where things should be (based on inferences of cosmic evolution). If these inferences are wrong, there might not be gravity where we think it is...
- We assume the universe would be smooth and featureless based on theories of cosmic evolution.