Tooth decay starts when the normal bacteria in your mouth combine with food and saliva to form a sticky substance called plaque that attaches to teeth. When you consume sugary food or drinks, the bacteria in the dental plaque mix with the sugars to make a mild acid. This acid attacks the hard outer layer of the tooth, called the enamel. If the dental plaque is not removed everyday by brushing and flossing, over time, the enamel gets soft and a cavity forms.
The dull spot on the tooth's surface may be decay. Brushing with a fluoride toothpaste and flossing may prevent it from becoming a cavity.
The decay is now a cavity. It has gone through the tooth's hard surface layer (enamel).
Now that the cavity has reached the softer layer of the tooth (dentin), it will get bigger faster.
If the cavity is not filled, it can cause bigger problems deeper in the tooth. This is why it's important to see your dental team regularly.