Here's a list of twenty important questions that remained unanswered by humankind, arranged into a timeline:

1. What is the origin of the universe, and how did it begin?
2. What is dark matter, and how does it affect the universe?
3. How did life originate on Earth?
4. What is the nature of consciousness, and how does it arise?
5. What is the true nature of reality?
6. Is there extraterrestrial life in the universe, and if so, where and in what form?
7. What is the cure for cancer and other diseases that afflict humankind?
8. What is the ultimate fate of the universe, and will it end in a "big crunch," "big rip," or "heat death?"
9. How can we develop a sustainable society that meets the needs of the present without compromising the ability of future generations to meet their own needs?
10. How can we reverse or mitigate the effects of climate change?
11. How can we achieve world peace and eliminate war and conflict?
12. How can we eradicate poverty, hunger, and inequality?
13. How can we ensure the ethical use of advanced technologies, such as AI and genetic engineering?
14. What is the nature of dark energy, and how does it affect the universe?
15. What is the relationship between the mind and the brain?
16. How can we achieve true artificial general intelligence, and what are the risks and benefits of such technology?
17. What is the mechanism behind gravity, and how does it fit into a unified theory of physics?
18. How can we preserve biodiversity and prevent the extinction of species?
19. How can we develop sustainable and scalable energy sources, such as fusion power?
20. How can we extend human lifespan and improve the quality of life in old age?

As for when and by whom these questions will be answered, it is impossible to predict with certainty. However, many of these questions are actively being researched by scientists, engineers, and other experts in various fields, and progress is being made every day. It is likely that some of these questions will be answered within the next few decades, while others may remain mysteries for centuries or even millennia.
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