Your first molar seems like having a decay that has started between the tooth. As long as you don't have any symptoms like pain, this requires a restoration or just filling.
Next Steps
Just get back to your dentist and get the restoration done as early as possible, as once the decay is in dentiny, second layer of tooth might spread faster.
Health Tips
You might experience sensitivity sometimes which is normal in deeper decayed part of the tooth. Which will subside once filling is done.