After the discovery of the cosmic microwave background, it was shown that high energy cosmic rays should be seriously attenuated. Nevertheless, several events have been detected. The sources of these cosmic rays and the production mechanisms are still a mystery and the uncertain evidence of the cut-off due to the cosmic background is today's main challenge. Something fundamental seems to miss in our understanding of the nature and propagation of cosmic rays and their sources. How can these particles be accelerated or produced at such energies? Which sources are responsible? Since their origin seems to be extragalactic, how can they travel over cosmological distances? In this paper we see why these questions are still unanswered and what are the most plausible answers so far.
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