Cavities are a usual problem with teeth. Some that you would know of because it has started to bother you, and others that won’t hurt you at all but the dentist may find when they do a complete exam and look at your xrays. Depending on the esthetic requirement and the severity of the cavity, a filling may be done using a silver-colored restorative material, or a tooth-colored material.
Some cavities may look small on the inside, but are actually a lot bigger inside, especially as it gets deeper into the tooth. They generally get worse with time, and so having them treated in the earlier stages will greatly increase the rate of success.