1."Science never solves a problem without making us wonder about new ones."

2. "Always question, Always wonder."

3. "Science is simply common sense at its best, that is, rigidly accurate in observation, and merciless to fallacy in logic."
A speech on that topic can begin by stating the scientific knowledge that modern day human civilization possesses. And compare it to the knowledge our ancestors possessed and conclude that many of the things we currently know were mysteries for our ancestors. Similarly, you can state that there must be many laws and principles that are currently mysteries for us and hence we can't explain motions of many celestial bodies.
You can also talk about the dark matter and god particle to invoke audience's interest in your speech.
For that topic the best way to conclude is by explaining Einstein's  words "nature is yet to reveal its true self to mankind" and he thinks that mankind is not yet ready to assimilate that knowledge.