‘It’s not the same thing. We don’t know what it consists of, elementally, but we know what it is. It’s dark energy and dark matter.’

‘No. That’s just a bloody hypothesis. It’s theoretical. It doesn’t exist outside some physicist’s head.’

Dad had been in the room, I remember, because I was always aware of him. He was reading his paper but he was listening. Mum might have been hovering, glasses hanging from a chain, but equally she might have been in the study, writing a paper.

Dad smiled because he saw something that I didn’t and he was enjoying it.

‘I disagree,’ Rory said. ‘It does exist.’

‘How do you know that? Has it ever been seen? Has it ever been detected?’

‘It could be a weakly interacting massive particle. If it interacted too weakly for detection. What with neutrinos and everything.’

‘It’s not a WIMP,’ I say.

‘No, but that’s not to say it doesn’t exist. It just means that it doesn’t interact with light or electromagnetism or any way that we have now of detecting its presence. But neither do thoughts or dreams. All we know is that ninety-five per cent of the Universe isn’t ordinary matter. So, it stands to reason that it’s something, whether it’s called dark matter or an elephant, it still exists. If, that is, you still want a standard model of cosmology, which you have to unless you want to throw Einstein out?’

‘That’s just bloody sophistry,’ I said, but I knew he was right. I left the room. In my bedroom I lay down, Psychedelic Furs playing at full volume on my ghetto blaster. Halfway through the first track, there was a knock and he spoke through the closed door.

‘I’m sorry. I don’t mean to win all the time. It’s – just – I know the answers. Sometimes,’ he said.

I didn’t reply.

He stood knocking at the door for some time. Calling my name but I remained silent.

‘You okay? Hello?’

‘You okay? Hello?’

I look up and see a face pinched at me in concern. Where am I?

I look down at the pavement before looking up at the face again. The expressions on these faces are all the same – guilt in some form. A woman who has lost her son to the streets or a man who has too much money and too little good to show for it. Even just a girl who has read too many books.

‘Where am I?’ I say.

The woman is maybe forty years old and looks like a person who has seen a lot and is afraid of very little. She has the face of a market trader.

‘Holborn,’ she says in a leathery voice.

Green Zone. ‘Time is it?’ I say, rubbing my eyes against the street lights. The darkness of the blue in the sky makes it hard to tell.