Can't remember what this story was called/where I read it. I am fairly sure it was part of a collection of other stories
Rough Plot (details are sketchy and possibly wrong so bear with me):
Young earth boy (maybe 12-15 or so) unknowingly boards an alien spaceship which is effectively some kind of giant arboretum with all kinds of animals, the boy is running off crying somewhere and somehow stumbles in there and doesn't realize he's actually on a ship until it takes off and he realizes things don't look right. Somehow this ship cloaks itself while landed (on earth) and acts as a kind of "sampler" ship, which is automated - i.e. apparently not piloted by anyone) and collects samples of the local fauna/flora/wildlife etc and stores it inside in its giant arboretum type thing.
Anyway, boy is confused, hopelessly lost, despairing, doesn't know what's going on, the ship is traveling through space, boy can't find anyone or anything. Ship eventually lands somewhere (I forget the details!) boy wanders around, I have the impression this ship is one of many of a giant network of such ships (earthlings are apparently oblivious to the existence of aliens), boy wanders around unhindered through all these automated (apparently unmonitored) facilities, looking for someone to communicate with, he can't find anyone, ends up boarding another ship (I think?) winds up on some alien planet with some kind of a city. City seems deserted i.e. nobody around, boy is hungry/thirsty, thinking he may die, etc.
Boy finds and enters building on this alien planet, discovers some kind of device with a sink, tries pressing the buttons but only succeeds in making some black slime come out (or something like that), keeps looking and can't find anything.
He finds a 'robot' which he manages to turn on and the boy is nearly dying of thirst/hunger, but apparently the robot is very smart and is able to determine that the boy needs food/water. The robot uses the same sink as before and creates water. The robot also takes a 'sample' of the boy, i.e. some of his flesh and then reconstitutes more of boy's flesh for the boy to eat (basically eating a reproduction of his own flesh, being a cannibal almost, but it's that or starve).
Gradually robot becomes smarter and smarter and begins communication with the boy, apparently the robot was designed to fulfill any/all needs of whomever is using it, the robot is unsure why everything seems to be deserted.
To cut a long story short now (since I'm rambling on and on):
- Apparently the previous occupants have ascended to a higher plane of existence leaving behind all their automation and infrastructure
- Robot becomes more and more human (female!) and basically takes the boy on a tour of the universe and shows him all the stuff
- Boy considers returning to earth, but apparently due to the relativistic nature of space travel, everyone he knew is dead
- Instead decides to see what the universe has to offer and play around with his new robot girlfriend
- At some stage I think he does contact Earth and tries to force them into being 'peaceful' by pointing some kind of alien armada at the planet or something.
Can't remember how it ends exactly, I know I've probably got things wrong/inaccurate, but hopefully if anyone can decipher my mad ramblings and have read it before can figure out the short story/book I'm thinking of.
There's your challenge: Name that Story!