What are white holes?
How do they affect spacetime?
A black hole has extreme gravity which bends the spacetime fabric, but what does a white hole do?
Also, what are they made of?
White holes are purely theoretical 'exit black holes' where matter sucked into black holes escape through white holes. They have not been observed in our universe and do not currently belong to mainstream physics. Thanks for the question though, and I hope I helped you a bit with it.