To add to this, they are detecting it based on how it has altered the orbits of other things over a long time, and not based on its direct gravitational effect.

To add to this, they are detecting it based on how it has altered the orbits of other things over a long time, and not based on its direct gravitational effect. Which gives you an idea what its orbit should be, but not where it is within said orbit. That gives them strip of sky to search through. A strip that unfortunately goes past some bright things - like the Milky Way.