LOS ANGELES, June 30 (Xinhuanet)-- From the composition of the universe to the upper limit of world population, big puzzles are challenging world scientists' wisdom, said the journal Science on Thursday.

    To celebrate its 125th anniversary, Science has taken stock of some of the most important, yet-unanswered scientific questions and delved into 25 of them for a closer look at just what we do and don't yet know about the universe.

    Questions like these show how far science has come in explaining the natural world, and they also fuel the discoveries of the future, said the Science editors.

    Ultimately they selected 125 questions for their list and focused on 25 that there was a chance of solving, or at least knowing how to approach solving, in next two decades or so. These 25 questions include:

    -- What is the universe made of? In the last few decades, cosmologists have discovered that the ordinary matter that makes up stars and galaxies is less than 5 percent of everything there is. What is the nature of the "dark" matter that makes up the rest?

    -- What is the biological basis of consciousness? In contrast to Rene Descartes' 17th-century declaration that the mind and bodyare entirely separate, a new view is that whatever happens in the mind arises from a process in the brain. But scientists are only just beginning to unravel those processes.

    -- Why do humans have so few genes? To biologists' great surprise, once the human genome was sequenced in the late 1990s, it became clear that we only have about 25,000 genes, about the same numbers as the flowering plant Arabidopsis. The details of how those genes are regulated and expressed is a central question in biology.

    -- How much can human life span be extended? Studies of long-lived mice, worms and yeast have convinced some scientists that human aging can be slowed, perhaps allowing many of us to live beyond 100, but others think our life spans are more fixed.

    -- Will Malthus continue to be wrong? In 1798, Thomas Malthus argued that human population growth will inevitably be checked, for example by famine, war or disease. Two centuries later, the world's population has risen sixfold, without the large-scale collapses that Malthus had predicted. Can we continue to avoid catastrophe by shifting to more sustainable patterns of consumption and development?

    Some of the questions were naturals, just really fascinating, others we chose based on how fundamental they are, and whether answering them would provide insights across several areas in science. Some were central to current social policy, for example relating to HIV or climate change, said the Science editors.

    "Today, science's most profound questions address some of the largest phenomena in the cosmos and some of the smallest. We may never fully answer some of these questions, but we'll advance our knowledge and society in the process of trying," said Donald Kennedy, Science editor-in-chief.

    "As Science celebrates its 125th birthday, we've recognized that an examination of science's outstanding mysteries also reflects its tremendous accomplishments," he noted in a statement.

    Founded by inventor Thomas A. Edison, Science debuted on July 3,1880 with 12 pages of articles on the possibility of electric-powered railroads. Issues over the following decades included articles by Albert Einstein, Edwin Hubble, Louis Leakey and other great scientists. Enditem