The article reviews dark matter (DM) candidates with masses below 10 GeV, extending down to 1 meV, which are motivated by hidden sectors such as Hidden Valleys. These sectors, featuring hidden forces and rich dynamics, have evaded traditional collider searches due to their weak coupling to the Standard Model (SM). However, they can still be detected through low-energy colliders or astrophysical observations of DM halos and stellar structures. The review focuses on mechanisms where the DM abundance is set through interactions with the SM, making it detectable in terrestrial experiments. The bulk of the article discusses new ideas for directly detecting these low-mass DM candidates through nuclear recoils, electronic excitations, or collective modes like phonons and magnons. The rich tapestry of materials and modes in condensed matter systems is reviewed, along with specific prospects for detection. The article also covers astrophysical, cosmological, and collider probes of light DM, including constraints from Big Bang Nucleosynthesis, Cosmic Microwave Background, large-scale structure, stellar evolution, and collider experiments.The article reviews dark matter (DM) candidates with masses below 10 GeV, extending down to 1 meV, which are motivated by hidden sectors such as Hidden Valleys. These sectors, featuring hidden forces and rich dynamics, have evaded traditional collider searches due to their weak coupling to the Standard Model (SM). However, they can still be detected through low-energy colliders or astrophysical observations of DM halos and stellar structures. The review focuses on mechanisms where the DM abundance is set through interactions with the SM, making it detectable in terrestrial experiments. The bulk of the article discusses new ideas for directly detecting these low-mass DM candidates through nuclear recoils, electronic excitations, or collective modes like phonons and magnons. The rich tapestry of materials and modes in condensed matter systems is reviewed, along with specific prospects for detection. The article also covers astrophysical, cosmological, and collider probes of light DM, including constraints from Big Bang Nucleosynthesis, Cosmic Microwave Background, large-scale structure, stellar evolution, and collider experiments.