Could there be totally dark galaxies

As matter coalesced into stars and galaxies, there must have been huge volumes of low density zones left behind. Within these zones gravity would continue to converge the remaining matter, (mostly hydrogen and helium from the big bang) perhaps not quite achieving densities sufficient for stars to form. If so, could such incomplete coalescence form totally dark galaxies; swirling clouds of hydrogen and helium not quite dense enough or lumpy enough for nuclear processes to switch on, and therefore invisible to our telescopes?

If you think about it, it is statistically pretty unlikely. A galaxy is quite a big thing - even a dwarf galaxy. For something that size to have its mass so uniformly distributed that it formed 'clumps', but that none of these ever became large enough to form a star, would be very odd indeed. Even if you started out with something like that, random perturbations and collisions within the dark galaxy would inevitably lead to clumps joining together and forming real stars, so your dark galaxy would also have to be very young.

No.
By definition that would not be a galaxy.
By the same token the intergalactic voids way very well contain higher than normal levels of matter in some areas.

But vast accumulations that condensed to form the hundreds of billions of stars in our galaxy, have always had the same gravity as it now has. Thus it is held together by gravity.

At some time in the distant future (perhaps hundreds or thousands of billions of years from now) all of the hydrogen will have been incorporated into stars and converted to helium and heavier elements. Thus new stars will not be able to form. The existing stars will use up their reserve of fuel then dim. That will be the fate of this and every galaxy. It will dim eventually becoming undetectable. It will still exist, but contain only the stellar remnants that have ceased to radiate energy.

Only to telescopes that observe solely in the visible spectrum.

Maybe.