Masses and Mass-To-Light Ratios of Galaxies

Is there more to a galaxy than meets the eye (or can be seen on a photo graph)? Many decades ago, Zwicky (1933) and Smith (1936) showed that if the Virgo cluster of galaxies is bound, the total mass must considerably exceed the sum of the masses of the individual member galaxies; i.e. there appeared to be "missing mass" in the cluster. As more data became avail able, the discrepancy persisted between masses of individual galaxies determined fr om optical rotation curves and the larger average galaxy mass needed to bind groups and clusters (e.g. Neyman, Page & Scott