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If you do not have any abnormality, then you need not worry. The skull does have a protuberance at the back, but in some cases there may not be. If this is something you always had, then you need not worry. However, if this has developed all of a sudden, then rush to your doctor and get a CT scan of head done.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
Well, I am 18 years old and have always noticed a weird sort of "dip" In the back of my head and noticed a few of my friends had it as well, but im sure it's not much to worry about, unless like the doctor said, "if this has developed all of a sudden".