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My understanding is that the difference between observed and predicted galaxy rotation rates is real, and the thing that is thought to cause it is given the nam

My understanding is that the difference between observed and predicted galaxy rotation rates is real, and the thing that is thought to cause it is given the name of dark matter. But that doesn't make it a placeholder! Any more, for example, than when people detected the effects of Mercury or Neptune on the orbits of other planets in the solar system, before they could see those planets directly, they could give them a name without being able to see them: yet those weren't placeholders, rather their effects on orbits was evidence of their existence. OTOH, maybe everything in science is a kind of a placeholder, but that sort of thing can get unpleasantly philosophical.

hanoz 10y ago
It seems to me there's a big difference between an explanation of an expected/obsevered mismatch which says there must be another massive object involved - and here's where to point your telescopes to look for it, and one which says well there must be some other mass of some form somewhere.
- conistonwater 10y ago
  For a pretty long time, for both those planets, they didn't know where to point their telescopes. This was also true for the outer planets, and Pluto too. A similar sort of thing, to a different/lesser extent, used to be the case with gravitational waves not having been directly observed despite other evidence. There were similar problems with the early discussions of atomic nuclei, electrons, neutrinos, neutrons, where all kinds of things were posited based on their effects only. I know it's quite a bit more difficult, and less direct, with dark matter, but still. (Really, I was objecting to the use of the word placeholder, because I found it unfair.)
samatman 10y ago
Mercury is visible to the naked eye. Consequently, its existence has been known since prehistory.
- conistonwater 10y ago
  You're right. I think I must have misremembered something I read about https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) But I think I got the example of Neptune right: https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregularities_in_Uranus.27s_orbit https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregular...

hanoz 10y ago

It seems to me there's a big difference between an explanation of an expected/obsevered mismatch which says there must be another massive object involved - and here's where to point your telescopes to look for it, and one which says well there must be some other mass of some form somewhere.

conistonwater 10y ago
For a pretty long time, for both those planets, they didn't know where to point their telescopes. This was also true for the outer planets, and Pluto too. A similar sort of thing, to a different/lesser extent, used to be the case with gravitational waves not having been directly observed despite other evidence. There were similar problems with the early discussions of atomic nuclei, electrons, neutrinos, neutrons, where all kinds of things were posited based on their effects only. I know it's quite a bit more difficult, and less direct, with dark matter, but still. (Really, I was objecting to the use of the word placeholder, because I found it unfair.)

samatman 10y ago

Mercury is visible to the naked eye. Consequently, its existence has been known since prehistory.

conistonwater 10y ago
You're right. I think I must have misremembered something I read about https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) But I think I got the example of Neptune right: https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregularities_in_Uranus.27s_orbit https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregular...