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Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained unless more matter is present than can be seen.
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The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.[2] Other lines of evidence include observations in gravitational lensing,[3] from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisions,[4] and from the motion of galaxies within galaxy clusters. [so mismatch in gravitational effect with Einstein General Relativity is the only reason why people think there “must” be dark matter out there. Imagining dark matter would help to keep Einstein General Relativity correct in its current form, in other words it’s a HACK!]
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Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists, intrigued by certain observations that do not fit the dark matter theory, argue for various modifications of the standard laws of general relativity, such as modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. These models attempt to account for all observations without invoking supplemental non-baryonic matter. [means without the need for the hypothetical, non-observable dark matter. And because the standard model can not explain gravity, dark matter or modified gravity theory has nothing to do with the standard model as stated in my original comment]