Ambient light always comes from
a light source, but indirectly
. Ambient light is defined as any light that is hitting stuff that has not come directly from the light source -- for example, scattered in the atmosphere, or bounced off the floor or walls or buildings.
This is why in space, shadows are
pitch black: there's nothing for the light to bounce off. A similar thing happens if you're in a room where all the walls are black: shine a light, and only the areas it hits will light up. Contrast this with a white room, where if you shine a light, it will bounce around like crazy off the walls and light up the shadows much more.