  Modern cosmology successfully deals with the origin and the evolution of the
Universe at large scales, but it is unable to completely answer the question
about the nature of the fundamental objects that it is describing. As a matter
of fact, about 95\% of the constituents of the Universe is indeed completely
unknown: it cannot be described in terms of known particles. Despite intense
efforts to shed light on this literal darkness by dark matter and dark energy
direct and indirect searches, not much progress has been made so far. In this
work, we take a different perspective by reviewing and elaborating an old idea
of studying the mass-radius distribution of structures in the Universe in
relationship with the fundamental forces acting on them. As we will describe in
detail, the distribution of the observed structures in the Universe is not
completely random, but it reflects the intimate features of the involved
particles and the nature of the fundamental interactions at play. The observed
structures cluster in restricted regions of the mass-radius diagram linked to
known particles, with the remarkable exception of very large structures that
seem to be linked to an unknown particle in the sub-eV mass range. We
conjecture that this new particle is a self-interacting dark matter candidate.
