Can we say cause/causes behind the world is/are eternal? There is one reality which is eternal and that is Nirvana. Nirvana had no beginning nor has an end yet it exists.
Nirvana and the world are closely linked up. The one who is in the world can attain Nirvana. 
If we say world has a beginning then Nirvana predates that beginning. Only Nirvana existed and then world came into existence. Which means World came from Nirvana as world is not unconditioned and before world there was only Nirvana.Which means world must have come out from state of Nirvana. But that is impossible.

Therefore can we at least conclude that cause or causes behind the world are without beginning?

update: I knew Buddha has said that it is undeclared whether whether world is eternal or not ,but people in this forum and elsewhere have declared Nirvana to be eternal.If Nirvana is eternal and permanent then we can say something existed for an infinite time in the past. Clearly Nirvana provides an eternal timeline to discuss upon. for example : In the infinite time in the past when Nirvana was there did world exist or not? Now  @ChrisW asked what did I mean by world ? By world I mean any reality in which experience of suffering can arise.
So to rephrase my question : Was there any point in the past when there was no suffering at all or has the suffering existed forever like Nirvana ? I know the gravity of this question as Buddha chose not to answer it. I dare to take up the question in order to provide some new insight to my fellow Buddhist men who think in order to believe in Buddha or Dhamma or Sangha one doesn't need to blindly believe in Buddha or Dhamma or Sangha. I believe I will also ultimately conclude that one should left such questions undeclared but I would like give one last try based upon the conclusion we have made so far like Nirvana is eternal. Given the infinite timelime ,if Suffering (or the world ) had a  beginning then what was before that beginning ? It was Nirvana. What was the cause of the suffering to arise at the beginning? Since Suffering is always conditioned and before beginning there was only Nirvana , clearly it means Nirvana was the cause of arising of suffering. But that is impossible. Because that would mean Nirvana is unstable, Nirvana is impermanent. Therefore conditioned suffering existed infinitely in the past independent of Nirvana. The cause of suffering (or the world) was impermanent but was/is a permanent feature of the world or existence or suffering which appears to be permanent. I can assure you that this question is not meant to lead you astray from the path but make us understand why we are not concluding what Buddha concluded, to know what is incorrect in our line of thinking.
Nibbana is an ultimate reality, but the 'world' is a concept. What you call the world is merely seeing, hearing, smelling, tasting, feeling, thinking. These are just experiences. There is no arising of experiences in Nibbana. 


  If we say world has a beginning


But we cannot say this as it has no basis in reality. So any conclusions or questions you come up with based on this assumption will only take you away from the reality. It is not in the spirit of Buddhism to entertain such questions. 

Be mindful of the questions arising in the mind that only serve to please your imagination. They will lead you astray from the Buddhist path.