Distance is always useful for hiding an astronomical body.
If your ships uses some type of hyperspace or subspace jumps to instantly move from one place to another, and if travel between solar systems and even other galaxies is common, some sort of accident or miscalculation during a jump through space could make it much longer than the normal jump through space.
Suppose that exploring spaceships normally jump to stars that are between 50 and 100 light years from Earth. Suppose this exploring starship wants to jump from the star system it just explored to the system next on his list, only 5.042 light years away, and for some reason which never happened before, the starship jumps ten million times as far, or 50,420,000 light years. Suppose that the spaceship only has enough fuel lief for a few short jumps, enough to find a nearby star system with a habitable planet and reach that. Suppose that the starship's only method of communication is to return to headquarters with their reports, and now they can't do that.
At the time of the story humans are exploring space 50 to 100 light years from Earth. And now an unprecedented and previously unimaginable accident has sent the spaceship 500,000 to 1,000,000 times as far as the zone of exploration.
Since the zone of space that includes the lost spaceship has about 125,000,000,000,000,000 to 1,000,000,000,000,000,000 times the volume of the present limits of human exploration, the time it takes human explorers to explore space as far out as the distance to the lost spaceship may be approximately 125,000,000,000,000,000 to 1,000,000,000,000,000,000 times as long as the time it took to explore out to 50 to 100 light years from Earth.
This is just a crude calculation, but it gives the general idea that a spaceship that jumps through space a great enough distance and can't get back might not be discovered by other explorers from Earth for many, many millions or billions of years, which would seem like "never" by human time standards.
If there is no form of faster than light radiation in your fictional universe, their astronomers, and the navigators on their ships, will have to use electromagnetic radiation, as our astronomers do, to study the universe.
The expansion of the universe can exceed the speed of light at great enough distances, because it is the space between objects that is expanding, and the objects are not technically moving.
So since distant parts of the the universe could have been expanding at many times the speed of light, and since light from those objects can only reach us aat the speed of light, and since the universe is only 13,000,000,000 or so light years old, there could be galaxies far beyond the most distant light that astronomers can detect, galaxies whose light might not reach Earth until billions or trillions of years.
Since the universe has expanded since the oldest light reaching Earth was emitted 13,000,000,000 years ago, the most distant objects we can see are now about 45,700,000,000 light years away. Thus we cannot now see or chart any astronomical object beyond about 45,700,000,000 light years away.
And that will have changed only minutely 10,000 years in the future, for example, when Earthlings might be exploring 1,000 light years from Earth. if some event causes a starship to make an unplanned jump 150,000,000,000 light years, and it is for some reason impossible to program a reverse duplicate jump, the crew will be lost.
If the radius that they can detect astronomical objects is 45,700,000,000 light years from their current position, and they have records of astronomical bodies within a radius of 45,700,000,000 light years from Earth, there will be a gap of 58,600,000,000 light years between the two sets of observations and they can never identify a known object to show the way back to Earth. And at that distance it could be trillions of years before explorers from Earth reach their region of space - which is the same as never by human standards of time.
Suppose that the starship is a colony ship taking many tens or hundreds of thousands of colonists and their equipment to a planet to be colonized, and the "accidental" distant jump far into space is actually caused by a dissident group among the colonists, who want to go to a distant world and found their own independent society, for the other colonists to do things their way. The dissidents could make certain that it was impossible for the other colonists to ever find their way back to Earth space.