When we go looking for sources of dark matter, we have lots of options of things that aren't lit up. For example looking at the distribution of the sizes of the stars, we expect there to be a lot of giant gas balls that weren't quite big enough to start fusion. We don't see them, but we've got good reasons to believe that they are there and can even make a good guess as to how much of them there are.
However when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies. This is when we are getting into the territory of imagining new kinds of particles that physics hasn't yet discovered.
Therefore the debate isn't about whether dark matter exists - it obviously does. It is about whether exotic forms of dark matter need to exist.
> when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies
The dynamics of galaxies are not the only place where non-baryonic dark matter is needed. Our understanding of baryogenesis in the early universe places limits on how much total baryonic matter there can be in the universe. But the expansion history of the universe requires much more matter than that (about 5 times as much IIRC).
When we go looking for sources of dark matter, we have lots of options of things that aren't lit up. For example looking at the distribution of the sizes of the stars, we expect there to be a lot of giant gas balls that weren't quite big enough to start fusion. We don't see them, but we've got good reasons to believe that they are there and can even make a good guess as to how much of them there are.
However when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies. This is when we are getting into the territory of imagining new kinds of particles that physics hasn't yet discovered.
Therefore the debate isn't about whether dark matter exists - it obviously does. It is about whether exotic forms of dark matter need to exist.