There are several mental disorders aside from ADHD that have unknown causes. A majority of mental disorders have unknown reasons.
The causes of clinical depression, anxiety, bipolar, obsessive-compulsive disorder, and even schizophrenia are all unknown. If one is to dismiss ADHD for this reason alone, then they would ignore all mental illnesses and conditions.
Just like ADHD, the causes of the previously mentioned disorders could be from a variety of different factors.
These factors include genetics, environmental factors, brain function, and prenatal exposures, and even brain injury. There are numerous medical mysteries, but when a majority of the population is experiencing the symptoms, how can people assume they are not real?
Research on ADHD continues today as a significant amount of the community is diagnosed with the disorder. With three million people diagnosed each year, denying the existence of ADHD because of its unknown origins would be ineffectual.