Access to the Expanse galaxy is limited to Captains level 30 and above and who also have acquired and installed an ITB on their ship.
Check out the Expanse Asteroid Locations.
Access to the Expanse requires a few costly investments. First, you need an ITB to enter it. That gets you access to the first three systems. Next up is an ALD, which get you a few systems farther. Beyond that, you need to survey systems before you can access them.