The general consensus I can find on what defines a galaxy is "Most astronomers agree on these basics: galaxies are massive, gravitationally bound systems of stars, gas and dust, with an important but poorly understood complement of dark matter. " and it’s not agreed because even with the measurements we have some that lack dark matter. So it is a good and open question even current theory lacks

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Ever see ship captains they eye ball it. He got outside looked up and said. “Yup that’s about right almost hit it”