When I see dark matter in the news I'm always reminded of the story of Vulcan.

In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.

Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.

Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.

When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.

Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?

MOND may not be the solution, but I'm still skeptical about dark matter.

There are multiple independent observations pointing toward dark matter:

1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg).

MOND only explains 1.

Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.

Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis.

Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.

100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better.

Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.

The constituent particles of nature are under no requirement to be apparent or achievable to you.

yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.