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It seems to me that the idea of dark matter grew from doppler measurements of the velocities of the outer stars of spiral galaxies. It was assumed that what was being measured was the orbital velocities around the galaxy's center.

The mass of a galaxy was assumed originally to be the sum of ordinary stuff (stars, debris, dust, black holes, etc.) and this mass's gravitational pull would not support the orbital velocities of the outer stars. Hence the idea of dark matter was proposed.

Is it possible that the outer stars are at or beyond escape velocity for an "ordinary" galactic mass and are simply moving outward rather than being in orbit? That they have been simply flung outward by some sort of pinwheel effect, perhaps off the ends of the inner galactic bar? In this case measurements of their velocities wouldn't be very useful in making assumptions about the galactic mass.

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    $\begingroup$ Check out the Bullet galaxy cluster. This interaction between two galaxies clinched the evidence for dark matter. $\endgroup$ – Lawrence B. Crowell May 28 '17 at 2:55
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If this was true, then, since we don't observe that the space between galaxies is full of ejected stars, it would require that all the galaxies we observe started ejecting stars in their relatively recent pasts. This would be an extraordinary coincidence, especially since many of them are causally disconnected from each other.

So, no, this is not a plausible cause.

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It mainly grew from doppler measurements of the spectral line of hydrogen at 21 cm, hydrogen which usually surrounds spiral galaxies and extend way farther than the "visible galaxy" (that is stars). If stars were leaving galaxies we would observe this in a lot of ways: flows of stars leaving, for example, or a big correlation between the age of the galaxy and the number of the stars or its spacial extension. We would observe that the older galaxies tend to have less stars than similar younger galaxies.

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