As humans continue to search for life in space, some experts speculate the so-called Great Filter theory lies behind why nobody is responding to our call.

Why has there never been contact? Civilisations likely destroy themselves before mastering communication, travel

Civilisations likely destroy themselves before mastering communication, travel Climate change, over-population may well be Great Filter for humans

Climate change, over-population may well be Great Filter for humans Distances communication must travel to connect with other lifeforms make it unlikely

Despite decades of scanning the skies, so far there has been total radio silence in the human effort to make contact with intelligent alien life forms.

But experts said in the end, humans would likely wipe themselves out before managing to make any contact.

Professor Peter Ward, astrobiologist at the University of Washington, said there were hundreds of billions of galaxies like ours, and therefore potentially billions of planets like Earth, that could sustain life.

"I absolutely believe that in our galaxy alone there are certainly other civilisations," he said.

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So why has there been zero contact? Professor Ward said the first problem was communication.

It's like intergalactic phone-tag — humans and aliens could be just missing each other's phone calls every time.

"So let's take every star within a radius of 100 light years, it's 100 years for one way," he said.

"And what if you bounce the signal again and then 200 years later it comes back and says, 'garbled please repeat'?

"[With] the distances and the difficulties of communication, are we within the lifespan of anyone ever getting a reply back?"

It's like intergalactic phone-tag — human's and aliens could be just missing each other's phone calls every time. ( Supplied: National Radio Astronomy Observatory )

Before anyone makes a connection, they're wiped out

However even before there is communication, the civilisations have to be on the same wave length — both literally and figuratively — at the same time.

That is — they have invented electricity, mastered sending radio wave communication and be able to beam it into space.

Professor Ward pointed out that in the millennia humans had been around, the capacity for intergalactic communication had only been achieved in the tiniest most recent window of time.

"There's 300 billion stars, we now know that virtually every star has a planet, so 300 billion times two or five or whatever you want — that's a lot of planets," he said.

"Just from those numbers, [there must be] some number of intelligent civilisation.

"So, the question becomes, 'What happened to them?'"

The Great Filter theory is that something leads to the destruction of civilisations, before they manage to master intergalactic communication and travel.

Professor Ward said climate change and over-population may well be the Great Filter that led to the end of human civilisation.

"If you look at any biological system, it doesn't care what species it is, when it overpopulates it begins to poison its home," he said.

"They say don't shit where you live. Well there gets to be a point where you can't help but do that."

Something lurking in our future 'will wipe us out'

Australia has a significant role in the Search for Extraterrestrial Intelligence (SETI) — and so far, we have come up empty-handed.

Carl Brusse, from the Australian National University, said the Great Filter theory could explain why intelligent lifeforms out there in deep space — and here on Earth — had not managed to let each other know about their existence.

Sometimes, the theory explains, it is factors beyond a civilisation's control, or they do something just plain dumb.

Civilisations could be wiped out by factors beyond their control — or they do something just plain dumb. ( Supplied: NASA/JPL-Caltech )

"We get to this point — we get past all the multicellularity, we get complex societies, we get technology, but there's something lurking in our future which will wipe us out," he said.

Professor Brusse said basically humans could have just missed the communications from other civilisations during the narrow window where they would have been at the same level, technologically, as humans.

"And then within a few decades and so forth they've wiped themselves out."

So should humans just give up in their search for intelligent life? Absolutely not, Professor Brusse said.

"We've only searched a tiny corner of the galaxy, and it's enough for us to start questioning whether our initial assumptions were correct and maybe downgrade the possibility of life," he said.

"But even if we're not finding anything, that's still telling us something.