How do you know that they're not using it? Perhaps they are using it for their internal Active Directory domain, which recommends in one scenario using a purchased, but un-used, registered domain name.
Perhaps it is used for staging purposes internally.
Perhaps the domain is tied up in a VC funding round and and is awaiting trademark registration and they don't want to run the risk of using the domain while the trademark is pending.
I'm not a lawyer, but I believe the only case you have is if they are infringing on your trademark. You may be able to launch a trademark infringement case against them to claim a domain name. But this has mixed success in the United States. See nissan.com or on Wikipedia.
I had a similar situation a while back. A 5-letter
.com domain name that matched our registered trademark exactly. I checked the Wayback Machine, saw that it was just personal domain in the late 90's and redirected to an IT provider now. I contacted the IT provider, and he explained that his client released the domain in the early 2000's but he had just held onto it.
We negotiated on a price over the next 2 months and settled on a very, very reasonable sum that was approximately 1/3 of which he had originally asked for the domain.
If you approach politely and with patience, you never know what you might get.