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Dark matter is hypothesized as the mass based on gravitational observations is much higher than that observed via measuring how bright a galaxy is.

Why could it not be that multiple stars in the distant galaxy are in a straight line, blocking one another from view and so the brightness from the hidden stars can't be measured? Similar to how during a solar eclipse, the sun is not visible behind the moon? Granted the moon is much closer than the sun and blocks out most of the sun, but cumulatively the brightness from multiple stars in the line of sight would not add up.

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    $\begingroup$ What mechanism do you suggest for making the stars line up with the Earth, and keeping them in line? $\endgroup$ – James K Nov 23 '19 at 8:24
  • $\begingroup$ And even if you did manage to keep them in line, you've still got the issue of gravitational lensing to consider. $\endgroup$ – antispinwards Nov 23 '19 at 11:38
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Nice thought, but as the saying goes, space is really big. Stars have a tiny angular size when viewed from even their nearest neighbours (except perhaps in clusters & galaxy cores), so not much starlight actually gets intercepted by other stars, relative to the amount of light that gets absorbed by interstellar (or intergalactic) gas and dust.

And even that absorbed light isn't actually lost: it increases the kinetic energy and the temperature of the gas & dust. So some of the energy is re-radiated at a lower frequency, typically as infrared or microwave radiation. By looking at the spectrum of that radiation we can determine its temperature, and use Doppler techniques to estimate the kinetic energy of the gas & dust.

When a star intercepts light from another star, the light energy isn't destroyed. Some of the light is actually reflected, the rest of the energy is assimilated by the receiving star's atmosphere, where it will contribute to the receiving star's own light emissions.

Astrophysicists take these things into account when evaluating the brightness of galaxies. Plenty of people have tried all sorts of alternative hypotheses to dark matter, but so far dark matter is the best explanation that we have.

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To add to PM 2Ring's answer, there's also the phenomenon of microlensing.

The point is, stars don't stay still on the sky. They move. Our Sun for example is moving around the center of the Milky Way. Therefore, even if two stars are completely aligned right now, they were not aligned in the past and likely will not be aligned in the future. In these scenarios, the first star should lens the light from the second star, and we should detect variations that indicate there's a star behind it. These microlensing events are being actively investigated since they're a great way to find exoplanets. If the missing dark matter is actually other stars, they should've been detected.

There are also much more technical reasons that have to do with why the fraction of baryons in the universe is about 5%. You can read more about this at, e.g., this section of the Wikipedia article on dark matter. This constraint also constrains the total number of stars there could be in the universe.

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