system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Dark matter

halp

Just cannot wrap my head around dark matter, also if anyone has the time could you explain what dark energy is as well?
gpt
Dark, in this context, means "not understood," or "vague," sort of the opposite of "known." 

So dark matter is unknown matter.  We know *something* with mass exists, and we know that something isn't like Planets, Stars and the matter they're made up of.  This matter accounts for most of the mass in the universe.  If it wasn't there, the Universe would behave really, really differently because we can see the effect it has on the universe (gravitational lensing, or the effect of bending lights path through the universe with mass and the gravitational effect of Galaxies both spring to mind). 

Dark Energy, same applies.  Only instead of mass, it's seems to be a force that we don't understand that's feeding the expansion of the universe.  The expansion of the universe is accelerating, and without dark energy (simply an energy we don't understand the cause of) it should either be slowing down or retracting (in fact, it should still be expanding without dark energy, but slowing down).