They can go one step farther now. They can directly observe gravitationaly lensing caused by dark matter. This is as much a direct observation as matter absorbing and emitting light. Not only have astronomers observed dark matter, but they are able to map where it is and its density:
You should look at some of the absurd mapping of dark matter in clusters. Small galaxies leave tails of gas as they revolve around usually two large elliptical galaxies. Some of the galaxies in the cluster shine at tiny fractions of the light frequencies of most of the cluster galaxies. How do we know they are linked? X-rays usually pervade the cluster showing that high energy stuff envelopes the entire cluster.
When trying to understand why scientists invent dark matter and stretching vacuums, one should start by analyzing the first principle of science. The Greeks were unable to invent an empirical science because they found no way to answer the problem of matter always changing itself. Fifteen hundred years later, Catholic monks adjusted Aristotle's system to fit their interpretation of the Bible. They invented the new concepts of being and essence. Their solution to the problem of matter changing itself was that the essence of substance is changeless. No one has ever detected any essence of anything. This assumption became the basis for western science. The definitions of physics, the measuring schemes - the mathematical constants, the methods - almost everything scientists do and think depends on their assumption that the properties of matter are not continually emerging.
None of the invisible things scientists populate their universe with (dark matter, a big bang, black holes, stretching vacuums, vacuums that stretch light etc) are needed if they could just believe what is visible - that the properties of all matter are changing relationally throughout cosmic history.
Only a biblical version of physics and a biblical cosmic history is supported by the only history that is visible as it happened, galactic history. We observe that galaxies were originally made of formless substances whose atoms shone at tiny fractions of the light frequencies of modern matter. We observe how the galaxies intrinsically grew, as the stars spread out, accelerated out as they continually changed their light clock frequencies. What we observe is the very things the biblical God says He does in unbroken continuity (in Hebrew - not English translations).
It is because the visible universe is a violation of the scientific creed, that scientists have filled it with magical, undetectable things. None of them are necessary if one just believes that ancient atoms clocked tiny fractions of the frequencies of modern matter, had different inertial properties and took up less volume of space.