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I'm an astrophysics student and I've been researching this topic and there is one point that keeps eluding me.

How did the scientific community realize that there had to be dark matter in the Universe?

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    $\begingroup$ One of the reasons that the is rotational velocity profile observed in galaxies was not matching the amount of visible matter. More generally, that's due to measured gravitational effects which imply the presence of more matter than what we see. Hence the name "Dark". More on that : wikipedia $\endgroup$ – Naptzer May 15 '18 at 14:46
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    $\begingroup$ I'm voting to close this question as off-topic because it shows insufficient prior research. $\endgroup$ – AccidentalFourierTransform May 15 '18 at 19:23
  • $\begingroup$ Nobody discovered dark matter in the same way that Rutherford discovered electrons: people have learned of a region in the data where something could be, and they call that “dark matter” $\endgroup$ – can-ned_food May 16 '18 at 14:41
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    $\begingroup$ @AccidentalFourierTransform Closing this question as off-topic is very clearly a disservice to the users. A question that has been viewed almost 5000 times, has 16 up votes and has 2 good answers with more than 20 up votes each should not be closed. Particularly because it will become a source for future researchers that may have they're questions voted off-topic for lack of prior research. $\endgroup$ – user3636673 May 24 '18 at 22:57
  • $\begingroup$ @JDOe I encourage you to take it to meta if you want to. $\endgroup$ – AccidentalFourierTransform May 25 '18 at 0:02
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Short answer: Due to a discrepancy between the density of matter which responds to electromagnetic radiation and the average calculated density of matter in the universe.

We have several ways to measure the average distribution of matter in the universe: for example, we can use the mass of an average star and the average number of stars in a galaxy, to get the average density of matter which is potentially visible (responds to electromagnetic radiation).

Alternatively, we can study the redshifts of different galaxies in a cluster, and thus determine the motion of each galaxy. This can help us calculate the gravitational force acting on each of the galaxies in the cluster, and hence we can find the mass of ALL matter in the cluster. Note how different this is from the previous method, which is for visible bodies only.

There are a few other techniques, but the essence is that the density of all matter is about 32% the critical density of the universe, but the density of matter which interacts with EM radiation is only 5% of the critical density. We call this constituents of this gap 'dark matter'.

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  • $\begingroup$ This answer is correct about how we currently know (or at least suspect to a level approaching certainty) that dark matter is there, but it doesn't really answer the question in the title or the text of the question, not withstanding that the OP has accepted it. The history is interesting and it (a) serves as an example of the scientific method at work and (b) should be shown to every newcomer who wants someone to explain why their idea is a non-starter. $\endgroup$ – dmckee Jun 24 '18 at 17:45
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Although it seems Fritz Zwicky can claim priority for having postulated dark matter (see freecharly's answer), Vera Ruben's observations of galaxies' rotation curves gave more evidence to the claim. The rotation curves she observed were flat ($\color{green}{\mathrm{green}}$ in the diagram below), showing that stars revolve much faster than expected based upon Newtonian mechanics ($\color{red}{\mathrm{red}}$ in the diagram below):

Observed vs. Predicted Galaxy Rotation Curve Dark matter postulate: Explains why there is much more mass in galaxies than the mass that emits light

Alternative theories of gravity (Modified Newtonian Dynamics (MOND), Weber's gravitational force, etc.): Explains galaxy rotation curves assuming we can "weigh" galaxies accurately based upon the light they emit.

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    $\begingroup$ Dark matter was first postulated in 1933 by the astronomer Fritz Zwicky, not by Vera Rubens. See my answer with links below! $\endgroup$ – freecharly May 15 '18 at 15:59
  • $\begingroup$ That's not a Keplerian prediction (nor should it be). A Keplerian prediction would be monotonically decreasing. However, since mass enclosed in an orbit in a galaxy increases with radius from the center, we wouldn't expect a Keplerian anyway, since Keplerian orbits assume a fixed amount of mass inside an orbit independent of radius from center. $\endgroup$ – NeutronStar May 15 '18 at 21:52
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    $\begingroup$ When I look at that chart, my first deduction is "the rotation speed of all objects in a galaxy tends to 200 km/sec with increasing range from the center", not "there's a specifically designed formation of dark matter that solves this problem that we just can't measure". What exactly has the dark matter approach over going for it? $\endgroup$ – Sudix May 16 '18 at 4:52
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    $\begingroup$ Obligatory xkcd. $\endgroup$ – kubanczyk May 16 '18 at 8:50
  • $\begingroup$ @Sudix That it solves this problem and several others (whilst introducing a different problem...) $\endgroup$ – Rob Jeffries May 16 '18 at 11:50
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Dark matter was first proposed already in 1933 by the Swiss astronomer Fritz Zwicky based on his observations of gravitational abnormalities in the Coma Galaxy Cluster using the virial theorem.

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  • $\begingroup$ It might be helpful to say explcitly that at the time 'dark' meant "not glowing in the bands that our (basically visible light) telescopes see". A meaning which has evolved over the decades as more an more cases of "Well, maybe it is [X]." have been experimentally shown to contribute far less than the needed total. Bonus points for an even moderately complete list of the hypothesis which have been tried... $\endgroup$ – dmckee Jun 24 '18 at 17:41

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