  The formation of tidal dwarf galaxies (TDGs) is triggered by the encounters
of already existing galaxies. Their existence is predicted from numerical
calculations of encountering galaxies and is also well documented with
observations. The numerical calculations on the formation of TDGs furthermore
predict that TDGs cannot contain significant amounts of non-baryonic dark
matter. In this paper, the first exhaustive sample of TDG-candidates from
observations and numerical calculations is gathered from the literature. These
stellar systems are gas-rich at the present, but they will probably evolve into
gas-poor objects that are indistinguishable from old dwarf elliptical galaxies
(dEs) based on their masses and radii. Indeed, known gas-poor TDGs appear as
normal dEs. According to the currently prevailing cosmological paradigm, there
should also be a population of primordial galaxies that formed within haloes of
dark matter in the same mass range. Due to their different composition and
origin, it would be expected that objects belonging to that population would
have a different structure than TDGs and would thus be distinguishable from
them, but such a population cannot be identified from their masses and radii.
Moreover, long-lived TDGs could indeed be numerous enough to account for all
dEs in the Universe. Downsizing, i.e. that less massive galaxies tend to be
younger, would then be a natural consequence of the nature of the dEs. If these
claims can be kept up in the light of future observations, the presently
prevailing understanding of galaxy formation would need to be revised.
