The art of composition is indeed an art. For lack of anything better, I'd recommend sticking to a pre-cast script, for instance:
- present the short summary as you stated in the OP, e.g. universe big, therefore humans insignificant.
- tell the reader how you are going to argue it, e.g. we will show that the universe is big beyond human grasp
- present the evidence
- state your reasoning, e.g. now that we know that the universe is so big, let's consider whether there is anything that a human can do to affect it
- repeat the short summary, e.g. thus the universe is too big for humans to be of any consequence to it.
Always present the facts in a convincing order so that the reader will come to the conclusion you want them to draw before you even state it. In other words, your arguments and conclusions should not come as a surprise. This is an essay after all and not a thriller novel.
To do that you need to plan how you are going to argue your essay. This is like building a house, and while the roof may look pretty, you need some walls to hold it in place. In your case, given that the universe is perceived as a far and remote thing, you may want to start with something closer to the average reader. Then, slowly broaden the view, present ever-increasing numbers, until you feel that you have conveyed the idea that the universe is really big.
Finally, be creative in your reasoning. For a reference, Bertrand Russell spends several pages in 'The problems of philosophy' arguing about whether we can really know the table in front of him. He starts with a clear description of the table, so that we readers are brought to think that perhaps we understand what this table is, and then, step by step, questions whether any of the features he presented is truly descriptive of the object. The result is just brilliant.