What Causes Hard Painless Knots On Scalp?

I have a few hard painless knots on my scalp. Have had them for as long as I can remember, probably at least the past 10 years if not longer. Started as just two at the top front part of my scalp but over the years the have grown, one rather largely showing almost through my hair. A couple more popping up as well. Just curious as to what they were or what could cause them?