We would be able to measure directly a magnetic field that has a high enough energy density to generate a gravitational field that produces the effects of dark matter
Magnetic fields polarise light. We would see it with ease. They also have no effect on most elements. There are a lot of reasons this is not the case, including observing the bending of spacetime caused by the dark matter. That is down to gravity.
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really_that_one t1_iyciyee wrote
We would be able to measure directly a magnetic field that has a high enough energy density to generate a gravitational field that produces the effects of dark matter