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Galaxies that are invisible in deep optical-NIR imaging but detected atlonger wavelengths have been the focus of several recent observational studies,with speculation that they could constitute a substantial missing populationand even dominate the cosmic star formation rate density at $z\gtrsim4$. Thedepths now achievable with JWST at the longest wavelengths probed by HST,coupled with the transformative resolution at longer wavelengths, are alreadyenabling detailed, spatially-resolved characterisation of sources that wereinvisible to HST, often known as `HST-dark' galaxies. However, until now, therehas been little theoretical work to compare against. We present the firstsimulation-based study of this population, using highly-resolved galaxies fromthe Feedback in Realistic Environments (FIRE) project, with multi-wavelengthimages along several lines of sight forward-modelled using radiative transfer.We naturally recover a population of modelled sources that meet commonly-usedselection criteria ($H_{\rm{AB}}>27\,\rm{mag}$ and$H_{\rm{AB}}-\rm{F444W}>2.3$). These simulated HST-dark galaxies lie at highredshifts ($z=4-7$), have high levels of dust attenuation ($A_{V}=2-4$), anddisplay compact recent star formation($R_{1/2,\,\rm{4.4\,\mu\rm{m}}}\lesssim1\,\rm{kpc}$). Orientation is veryimportant: for all but one of the 17 simulated galaxy snapshots with HST-darksightlines, there exist other sightlines that do not meet the criteria. Thisresult has important implications for comparisons between observations andmodels that do not resolve the detailed star-dust geometry, such assemi-analytic models or coarsely-resolved hydrodynamical simulations.Critically, we demonstrate that HST-dark sources are not an unexpected orexotic population, but a subset of high-redshift, highly-dust-attenuatedsources viewed along certain lines of sight.

Disappearing galaxies: the orientation dependence of JWST-bright, HST-dark, star-forming galaxy selection