The mass of a system inferred from the motions of its member galaxies is different (usually smaller) than that inferred from the observed radiation. This discrepancy is generally attributed to the presence of dark matter in the system. Such a model requires that the system galaxies (the source galaxies) merge with one another to form a single product galaxy. The predicted time-scale for this event manifests itself as a ratio of the number of product galaxies to the number of source galaxies in a given volume of space. Studies of binary-supergiant galaxies and compact groups of galaxies suggest that the predicted product-to- source ratio may be smaller by a factor exceeding ten than that actually observed. Analyses for these systems are to be refined by means of more sophisticated techniques whose application is warranted by new data. Tests of the universality of the effect require statistical homogeneous samples of different types of systems. These are to be identified through application of dendrogram techniques.
