I discuss the role of a hidden sector consisting of mini charged fermions (MCPs) and massless hidden photons in the expansion history of our Universe. The measurement of the anisotropies of the cosmic microwave background (CMB) by Planck and of the relic abundance of light nuclei produced during big bang nucleosynthesis (BBN) are now precise enough to predict the properties of such a hidden sector through the amount of extra relativistic particles ($N_{\text{eff}}$). I present interesting parameter regions for MCP masses between $\sim$
100 keV and $10$ GeV and minicharges in the range $10^{-11}-1$. The presented results can be used to put bounds on MCPs from the current data and they are also a valuable indicator for future experimental searches, should the hint for dark radiation manifest itself in the next release of Planck's data.
