Markings on my phone

I’ve had this phone for about 2 years now, and recently I was cleaning the sides of my phone when I realised that there were some very well lined up dots on the side of the phone, it looked like the dot were from the inside as I couldn’t feel the bump of the dots when I slide my fingers across them, I tried to use my fingernails the gently scratch it, it didn’t come off too, I at first thought it was from my phone case, but when I looked at the phone case interior sides, it didn’t have the same circle dot thing that would make those dots on the phone. I’d like to know what those dots are. Thx a lot!