Endless and indefinite are slightly different.
- An endless state will (of course) never end, under any circumstances. It goes on forever.
- An indefinite state is a state that has no known end point, but could end eventually (or may not). Its end time is not known or not defined (thus, in-definite).
Once the sun goes out, there will be endless darkness in the sky.
The will never be light in the sky again, without the sun. This darkness will go on forever.
I don't know when I will get money to fix my car, so I need to use your car indefinitely.
There is no fixed end point to borrowing your car. I need your car until I get more money, which may or may not happen at some unknown point in the future. I might need to borrow your car forever, or I might need it for a limited time. I don't know, and cannot define the length of time I will need it.
In short, indefinitely allows the possibility of an end (at an unknown time), but it does not promise an end.