Skaro is not in our Milky Way galaxy and is located in the Seventh Galaxy aproximately 480 million light years away from Earth.
In Genesis of the Daleks (1975) it is stated that Skaro is situated in the "Seventh Galaxy" and that, according to the Kaleds, Skaro was the only planet capable of supporting life in all of the Seven Galaxies indicating a small cluster of galaxies which would make them eligable for inclusion in the Hickson catalogue of compact groups. To put this into perspective, our Milky Way galaxy is a member of a local cluster of 34 galaxies.
Copeland's Septet is a close group of seven galaxies that lies about 480 million light-years away in Leo, discovered by Ralph Copeland in 1874 and the seventh member of Copeland's Septet is NGC 3754, a spiral galaxy (Members: NGC 3745, NGC 3746, NGC 3748, NGC 3750, NGC 3751, NGC 3753, NGC 3754).
NGC 3754: known locally as the Seventh Galaxy
In 1982, the Canadian astronomer Paul Hickson, began compiling a catalogue of compact groups of galaxies which he had identified. These groups became known as the Hickson Compact Groups of galaxies, and Hickson published his list of 100 groups in his 'Atlas of Compact Groups of Galaxies.' In the first image showing Copeland's Septet, there is also "Hickson 57" to show the catalogue entry of this compact cluster.
It is unclear why the Kaleds referred to their home galaxy as the Seventh Galaxy and not the "First" (or even a proper name) but it is possible that this just may be a quirk of the Tardis translation matrix.