  We present the Australian Square Kilometre Array Pathfinder (ASKAP) WALLABY
pre-pilot observations of two `dark' HI sources (with HI masses of a few times
10^8 Msol and no known stellar counterpart) that reside within 363 kpc of NGC
1395, the most massive early-type galaxy in the Eridanus group of galaxies. We
investigate whether these `dark' HI sources have resulted from past tidal
interactions or whether they are an extreme class of low surface brightness
galaxies. Our results suggest that both scenarios are possible, and not
mutually exclusive. The two `dark' HI sources are compact, reside in relative
isolation and are more than 159 kpc away from their nearest HI-rich galaxy
neighbour. Regardless of origin, the HI sizes and masses of both `dark' HI
sources are consistent with the HI size-mass relationship that is found in
nearby low-mass galaxies, supporting the possibility that these HI sources are
an extreme class of low surface brightness galaxies. We identified three
analogues of candidate primordial `dark' HI galaxies within the TNG100
cosmological, hydrodynamic simulation. All three model analogues are dark
matter-dominated, have assembled most of their mass 12-13 Gyr ago, and have not
experienced much evolution until cluster infall 1-2 Gyr ago. Our WALLABY
pre-pilot science results suggest that the upcoming large area HI surveys will
have a significant impact on our understanding of low surface brightness
galaxies and the physical processes that shape them.
