Before going to post back let me be clear on what is a View State.
View state is actually a client side state management technique in ASP.NET to get values or retain the values in post back or round trip.
Now if you ask where data is stored while post back, its clear that :
Its stored in client side within hidden control or field of a html form named “__VIEWSTATE”
More References to clear your question is listed below :
Stackoverflow Related Question