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"Dark matter is so called because we can detect its gravitational effects ... "

This raises an interesting question. How do we know that Dark Matter is needed to explain the gravitational effects that we observe, as opposed to simply not having accurately estimated the mass of the matter we are observing ?

I was listening recently to a discussion of dark matter by experts in the field and they were asked to explain to the audience how they *know* that dark matter exists, if we can't observe it. They gave an example (from real observations) of a galaxy some 400 million light years away and the fact that the stars orbiting around the outer "rim" of that galaxy are orbiting far too fast - they should be flung out of orbit, unless there is enough gravity to hold them in their orbits.

But the galaxy isn't heavy enough to have that gravity, so it *must* be dark matter.

How do they know the galaxy isn't heavy enough ? Why, from observing it's gravitational effects of course. But apparently *not* the effect of holding on to stars that are whizzing around faster than expected ? *That* effect must be from this dark matter stuff. Huh ?

How can we be so certain that we aren't simply under-estimating the mass of the matter we can see ?

It's not as if you can nip over over there and put all the matter within that galaxy on some scales to verify the mass calculation.