Manifold: Space by Stephen Baxter
This is the second book in the Manifold trilogy. Manifold is not one story so long it requires three volumes to hold it. Rather, it is more like three alternative histories for the same characters.
In this book's variant of reality, the Japanese have colonized the Moon. A scientist there, Nemoto, detects activity in the asteroid belt. In this version, Malenfant is an ex-astronaut businessman. A probe is sent to take a look. The probe encounters a huge spaceship and is captured by the ship. The spaceship does not respond to scientists' attempts to communicate with it.
The aliens seem to be robots and the Japanese call them Gaijin. They're building something in the asteroids.
It is believed the spaceship came from Alpha Centauri, so Malenfant wants to send an expedition there. On this expedition, Malenfant finds a portal that transports him across interstellar distances. He uses portals to pop from one place to another. Along the way he encounters a society of robots. Through the robots he attains great life extension, allowing him to journey all the further. He wanders long and far.
Meanwhile, Malenfant's extended disappearance has an impact back home. Nemoto discerns the existence of the robots and acts to deal with them as enemies. This is a long and complex subplot, extending through many changes for the Moon and Earth.