Several times during your journey, you'll see the names Nada and Polo referenced alongside Atlas as an alternate path. Here's how you find Nada and Polo.
After you get Antimatter from Specialist Loch to make your first Warp Core, they'll mention Nada and Polo. The game makes reference of this in space as well, saying you can go find them if you choose. After about the third or fourth warp jump, your scanners will pick up on an “Anomaly” in space and mark it on your HUD. Travel to the anomaly and enter the space station you find there to meet a Gek and a Korvax. Make sure you have enough fuel for the return trip, as the station is often far from a system's cluster of planets.
The Korvax Nada will give you three options. If you're only interested in exploring freely without any worry for any of the overarching paths, you can ask for resources. If you want guidance following the Atlas path or if you want to get back on it after refusing, they'll show the way to the next Atlas station. If you want to just get to the center of the universe, they'll show you a route to a black hole, which shortens your journey. Note that you can find Nada and Polo multiple times, so you can choose more than one option from Nada in a single playthrough.
The Gek Polo only wants to see the data on your interactions with other aliens. Satisfy them and they'll give you an Atlas Pass Blueprint, which creates an Atlas Pass that can unlock certain doors and containers. The Atlas Pass created by the Blueprint you get when you talk to Polo the first time will unlock cylindrical containers with Suspension Fluid or Electron Vapor, both items in the chain towards creating a Warp Core. The doors unlocked with the Atlas Pass v1 will often lead to Suit Upgrade terminals.