Kathryn M. Zurek reviews dark matter (DM) candidates with very low mass, ranging from $ m_{\chi} \gtrsim 1~\text{meV} $ to $ m_{\chi} \lesssim 10~\text{GeV} $. These candidates are motivated by hidden sectors like Hidden Valleys, which have weak interactions with the Standard Model (SM) and thus evade traditional collider searches. They can be detected through low-energy colliders or astrophysical observations of DM halos and stellar structures. The review discusses mechanisms for setting the DM relic abundance, including thermal freeze-out, freeze-in, and interactions with the SM. It also covers astrophysical and cosmological probes, such as Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background (CMB), and Large Scale Structure (LSS), as well as collider experiments. The focus is on direct detection methods, particularly nuclear recoils, electronic excitations, and collective modes like phonons and magnons. The review highlights the importance of understanding DM interactions with the SM and the role of hidden sectors in setting the relic abundance. It also discusses constraints from BBN, CMB, and stellar cooling, which limit the possible DM masses and interactions. The review concludes that low-mass DM candidates can be detected through terrestrial experiments, particularly in condensed matter systems, and emphasizes the need for novel detection methods due to the weak interactions of these particles.Kathryn M. Zurek reviews dark matter (DM) candidates with very low mass, ranging from $ m_{\chi} \gtrsim 1~\text{meV} $ to $ m_{\chi} \lesssim 10~\text{GeV} $. These candidates are motivated by hidden sectors like Hidden Valleys, which have weak interactions with the Standard Model (SM) and thus evade traditional collider searches. They can be detected through low-energy colliders or astrophysical observations of DM halos and stellar structures. The review discusses mechanisms for setting the DM relic abundance, including thermal freeze-out, freeze-in, and interactions with the SM. It also covers astrophysical and cosmological probes, such as Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background (CMB), and Large Scale Structure (LSS), as well as collider experiments. The focus is on direct detection methods, particularly nuclear recoils, electronic excitations, and collective modes like phonons and magnons. The review highlights the importance of understanding DM interactions with the SM and the role of hidden sectors in setting the relic abundance. It also discusses constraints from BBN, CMB, and stellar cooling, which limit the possible DM masses and interactions. The review concludes that low-mass DM candidates can be detected through terrestrial experiments, particularly in condensed matter systems, and emphasizes the need for novel detection methods due to the weak interactions of these particles.