Diminished Value is the difference in market value between a car that's been in an accident and one that hasn't. The age and mileage of the car can reduce (or eliminate) this difference, meaning that a 7-year old car with 100k miles will be worth the same regardless of whether it's been in an accident or not (the loss in value due to normal wear and tear is typically much higher then the loss due to the accident).
There is a formula that you can use to calculate the diminished value, which starts at 10% of the market value and goes down based on mileage and type of damage.
If you have evidence that the actual change in market value is more than what the standard formula gives, I would talk to a local attorney that has experience in insurance claims and see if there's any possibility of getting a diminished value claim, but unless the car was of high value before the accident, it may be difficult. If an attorney contacts the insurance company they may be willing to pay the claim just to avoid the hassle.