This sounds like you might've had a corrupted download from Steam, or have extra files in your /Stellaris/ folder.
This is our standard "something is broken" fix; since you don't have any mods, skip to step 2. :)
After each of these steps, reload the game and see if the issue persists (you may need to start a new game for some issues):
1. Disable all your mods, and ensure you're using a mod playlist with no mods enabled.
2. Verify your game files: (in Steam) Right-click Stellaris, Properties, Local Files, Verify Game Files
3. Manually delete /Stellaris/ and redownload: (in Steam) Right Click Stellaris, Local Files, and click Browse. Delete the contents of the /Stellaris/ folder that opens (do *not* delete the /Stellaris/ folder in /Documents/, as it contains your save files and custom empires). Then repeat Step 1.
If none of these things work, then go to ...