Moons around 'rogue planets' could sustain life — no solar system required

Light, as such, isn't a prerequisite for life - but energy is. Organisms need energy to live, and that energy has to come from somewhere. For most organisms on Earth, that energy comes ultimately from the Sun, even if the energy travels via intermediaries (for example, plants use chlorophyll to convert sunlight into chemical energy, animals eat plants to convert them into energy, and humans eat plants and animals).

There are some organisms on Earth that get their energy from other sources, such as bacteria that live near geothermal vents near the bottom of the oceans, which obtain their energy from the heat of the Earth's core (which is ultimately sourced from the gravitational force of Earth's mass compressing itself).

However, all organisms need energy - and the biggest and most accessible sources of energy in our universe are stars. A planet that's not near a star is going to have a lot less available energy for its biological organisms than a planet that's near a star: geothermal heat and radioactive decay don't provide anywhere near as much energy as a massive ball of fusing hydrogen. Therefore, planets that are not near stars are a lot less likely to have life - and, if they do have life, it will be limited in its capabilities.