The book is ~20 years old. Its plot is a gigantic spaceship (builders unknown) that passes through our solar system and a delegation from Earth is sent and discovers multiple species of aliens. There was a follow up book that explored a space in the center of the ship that was hell like.
This is Marrow, the first of the Great Ship novels by Robert Reed.
When a jovian-sized, artificially-created structure enters the galaxy, a society of technologically advanced humans (capable of interstellar flight and functionally immortal) are the first to intercept and investigate it. Finding it to be an intergalactic ship, they decide to convert it into a cruise ship, inviting alien races to join them in its massive, uncharted interior as it makes a slow circumnavigation of the Milky Way.
A key plot point is that a group of explorer get stuck on a planet lurking at the centre of this ship. It has energetic volcanism and is really quite an unpleasant environment.
After thousands of years, with over 200 billion creatures living in its upper levels, a group of explorers discover a planet hidden in the core of the Great Ship. As they explore it, however, an ionic blast cuts them off from the rest of the ship and destroys much of their technology. Because this planet, Marrow, is slowly expanding, the explorers reason that a new bridge can be built in another 5,000 years. They thus begin a civilization on the surface of Marrow.
This could be the Rendezvous with Rama series, depending on what you mean by hell at the centre.
- The spaceship(s) have picked up multiple species as they pass through various systems
- The destination is a kind of heaven, and has some religious connotations. Not all of the alien individuals or species qualify for ascension
- The experiences of the alien races on the ship can be seen as a metaphor for hell
As I remember, another book with a similar plot of a giant spaceship entering the solar system and astronauts visiting it was James White's All Judgement Fled 1966.
Science fiction writer Mike Resnick called the novel "Rendezvous with Rama done right."
Possibly Colin Greenland's Take Back Plenty (1990)
The giant spaceship is called Plenty. If I recall correctly, it isn't inhabited when the delegation from Earth gets there, but it soon becomes populated by an assortment of alien species from the (non-realistic) Solar System and sets off for distant stars. The sequel, Seasons of Plenty, explores the strange interior of Plenty, which has some horrible elements. In the third and last book, Mother of Plenty, Plenty arrives at a star inhabited by hostile aliens, and the protagonist must fight to save herself and her friends.