The Tarkans were an extremely hostile civilization in the Delta Quadrant. The USS Voyager was warned by the Travellers to avoid Tarkan space, a detour that added a month to Voyager's journey to the Alpha Quadrant. The Tarkans were said to take intruder's ships as trophies and then strand the crews on a moon. (VOY episode: "Darkling"; VOY - Strange New Worlds I short story: "Monthuglu")
No one knew why the Tarkans did this, as they had their own ships. But Kes accidentally read the mind of one and the reason came to light. As it turned out, the Tarkans were afflicted by the Tarkan wasting syndrome, a devastating plague that ravaged the Tarkan worlds for generations. The crews of the Tarkan ships, unable to return home, felt compelled to capture other ships to give themselves room to reproduce and expand their population. Since the disease could lie dormant for decades, they felt the need to spread out their population into as many separate small groups as possible, to minimize the losses if an outbreak occurred.
In an alternate timeline, Kes was close to finding a cure for the disease in hope to end the piracy. Later on, the Tarkan joined several other species in defending the Vostigye home system from an attack by the Voth. (ST - Myriad Universes novel: Places of Exile)