Hello,
From the picture of the lower teeth, it seems like hard deposits (calculus) which develops over a longer period of time , if the area is not properly cleaned. These calculus seems to have broken in bits giving gaps in between teeth, making it look like tooth decay.
Due to prolonged improper brushing techniques, such deposits occur. While brushing we usually scrub the outer teeth and forget to brush on the backside of teeth.
Next Steps
Get your teeth cleaned(scaling) by your nearby dentist, you don’t need a specialist for cleaning of teeth. And ask your dentist to teach brushing techniques, which will help you maintain your oral hygiene.
Health Tips
Regular follow ups every 3 or 6 months would go a long way in maintaining oral health.