☀️A Dark Vulcan

Vulcan was "discovered" by Lescarbault in 1859, in the sense that he saw something that he took to be the planet hypothesized by Le Verrier earlier that year. Le Verrier was already famous for his 1846 prediction of the existence and position of Uranus:

That prediction was based on observations of the planet Neptune. Neptune was not behaving as Newtonian dynamics predicted it would, not unless there was an unobserved planet like Uranus. A few days later, Uranus was discovered by Galle, who saw it roughly where Le Verrier had said it would be.

In 1859, Le Verrier hypothesized that observations of the planet Mercury might be similarly explained by there being a planet between Mercury and the sun. And Le Verrier was sure that Lescarbault had discovered Vulcan. But whatever Lescarbault had seen, it was not Vulcan. In the following decades, many observations of the absence of Vulcan were made. And while some astronomers claimed to have seen Vulcan, there seemed on balance to be no such planet. Now, the motion of Mercury was eventually explained in 1915, by Einstein. But my question is this:

Why was the balance of opinion before then not for the existence of a dark Vulcan?

Physicists do say that there is dark matter in the universe. Its existence is said to explain modern astronomical observations: the stars do not behave as Einsteinian dynamics predicts, unless there is dark matter. In other words, those observations contradict Einsteinian dynamics, to some extent. Incidentally, Einsteinian dynamics has also been contradicted by quantum-mechanical observations of Bell's inequality, to some extent. And particle physics is increasingly reminiscent of celestial epicycles (which had, much earlier, been hypothesized to explain other astronomical observations, before those observations were explained by gravity). But my question is:

Why did nineteenth-century physicists not hypothesize a lot of dark matter in the universe?

That hypothesis is relatively simple. And if there was a lot of dark matter in the universe, then most of the dark matter in the solar system would be clustered around the sun, possibly in the form of a dark planet between Mercury and the sun. And it is not as if physicists would not formulate such a hypothesis.

Did something happen to physics in the twentieth century?

Well, science did become more of a cultural phenomenon in the twentieth century.