The "great darkness" here refers to the Coalsack Dark Nebula. This is translated as "all that dark" by Roger Pulvers.
Speaking of which, here's Roger Pulvers' translation, from the 1996 "Night On The Milky Way Train" bilingual edition published by Chikuma Shobou:
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'Campanella,' said Giovanni, sighing deeply, 'we're alone again. Let's stay together till the ends of the earth, okay? If I could be like that scorpion and do something for the benefit of all people, I wouldn't care if my body burnt up a hundred times over.'
'Me too,' said Campanella, his eyes welling with the clearest tears.
'But what is real happiness, Campanella?'
'Don't ask me,' he answered dreamily.
'We'll keep our spirits up, won't we?' said Giovanni, taking a deep breath and feeling a new strength gushing through him.
'Hey, there's the Coal Sack!' cried Campanella, pointing to a spot in the Milky Way and leaning back as he did so. 'It's a hole in the sky!'
Giovanni shivered in fright as he looked at the Coal Sack. It was a huge black gaping hole in the river, and the longer he stared and squinted into it, the more his eyes smarted and he couldn't tell how deep the bottom went or what was down below it.
'I'm not scared of all that dark,' he said. 'I'm going to get to the bottom of everything and find out what will make people happy. We'll go together, Campanella, as far as we can go.'
'Yes we will, Giovanni. Oh look over there,'cried Campanella, pointing to a distant field. 'That is the most beautiful country I have ever seen. Everybody's there. That's the real heaven. Look, my mother's there too. Look!'
Giovanni looked, but what he saw was all milky white and blurry, not at all like what Campanella was describing. He felt indescribably lonely as he peered out, catching sight only of two telegraph poles on the opposite bank , their red crossbeams linked, like arms.
'Campanella,' said Giovanni, turning toward him, 'we're going to stick together, okay?'
But there was no Campanella where Campanella had been sitting, only the black shining velvet seat.
Giovanni bolted up like a rocket, leaning far out the window so that he wouldn't be heard as he screamed into the sky, pounding his chest hard and crying out with a throatful of tears.
Everything seemed to go black all at once.
[Bolded and spoilered mine.]
Again, I don't think the reference has been executed well. Without the original context, it makes you think of things that go bump in the night instead of literal darkness or the void.
Updated
Still...?Still not here?
No matter where...
I'm not even scared of those loud noises in the dark anymore.
Why isn't it coming?
I'm sure we're going to search for everyone's true happiness.