The unexpected discovery was made by a team exploring a 12 billion-year-old cluster of stars in the Milky Way called M22.
It contains hundreds of thousands of stars and can be seen with the naked eye as a hazy patch of light.
Scientists at the International Centre for Radio Astronomy Research now believe there could be as many as 100 black holes inside the cluster.
Dr James Miller-Jones, who co-wrote a report that appears in the journal Nature, said: "Simulations of how globular clusters evolve show that many black holes are created early in a cluster's history.
"The many black holes then sink towards the middle of the cluster where they begin a chaotic dance. Most are thrown out until only one surviving black hole remains.
"We were searching for one large black hole in the middle of the cluster, but instead found two smaller black holes a little way out from the centre, which means all the theory and simulations need refinement."
Black holes are formed when a massive star reaches the end of its life and collapses in on itself.
They are so dense that even light cannot escape them.
The M22 cluster, which can be found in the constellation Sagittarius, was discovered almost 350 years ago.
It is 10,000 light years away, so astronomers on Earth only see it as it was 10,000 years ago.
The research involved scientists from the University of Southampton, as well as universities in the United States and Australia.