~に興味がある (have interest in)

The pattern Xに興味がある or Xに興味を持つ, 'to have interest in X / be interested in X.' に marks the target of interest. With がある, 興味 is the noun-subject of existence; with を持つ, the speaker/subject actively 'holds' the interest — both mean essentially the same. Add ~ている for the resulting/ongoing state of having that interest. Among the most common ways to express interest in a topic.