I'm pretty much rehashing what everyone else has said, but what is the point of a discussion if not to repeat the same few points that everyone agrees with in many different ways?
There are many contributing factors to how many votes an answer might get. The most obvious and influential being whether or not the answer is correct and complete. However, it is far from unheard of that a masterfully made answer with just the right information presented in an easy to understand way might get few or even no votes.
Vote-hunting (that's a new term I just made up) is a skill; sadly one I am not very adept at, but I am an adept pattern recognizer so I see the trends. Beyond being correct and easily understood, it particularly helps if the question has a simple title; one most users could understand and most non-physicists would think "hey, that sounds pretty cool!" That said, too simple and it's likely to be ignored by everyone. The question you gave as an example is perfect for this. "How exactly does gravity work?" That type of title will definitely bring the viewers. The other thing that helps is if a question has a lot of up votes. That tells users that most people like the question and find it interesting. This makes them more interested in seeing what the answer to it is.
Once they've clicked in, you want the viewer to stay. To that end, the question needs to be well worded, easily followable, and interesting to novices and at least undergrad level as well. You might consider editing the format or grammar of the question if you feel it needs improvement in those areas but remember do not make any substantive changes to the content of the question unless it is your own. Also, consider improving the tags to include not only subjects mentioned in the question, but also subjects you know would have to be mentioned in any correct answer.
This will all lure in users and make them wanting to find out the right answer. Right off the bat, users usually upvote the accepted answer before moving to others. So if yours is accepted, great! If not, this is an uphill battle; your answer will need to be really good. I find that when there are a lot of answers at around the same vote level (say 5 with 0 or 1 vote), a really good answer can get muddled and lost. At this point, adding pictures really helps bring votes to you. If you can set your answer apart from the others visually, people are more likely to read yours, especially if all the answers are on the long side.
Medium to long answers usually are more correct than short answers and so they tend to be read first and upvoted. But too long and nobody wants to take the time. This is also true with math-heavy answers, answers that aren't segmented with short, manageable paragraphs, or answer that start with things like "This is just my opinion, I really don't know for sure..." Sometimes, a lot of math is called for, so then it's fine.
Furthermore, try to make it fun to read. Put a little piece of unexpected humour right in the middle of your answer, use wild and/or silly scenarios for thought experiments, be creative. Also, images. Let me restate that. Images. When you have a nice image with a splash of colour, that makes people want to read it. If you can find an image that could pretty much answer the question by itself and you pair it up with a nice written explanation that adds additional info for the more curious readers, then you're gold. Answers like that might win you the populist badge.
Avoid the use of this:
Edit:
It's all well and good for telling people that you fixed a problem or added in missing info, but it's exhausting for a reader. It may get you points from a few expert users, but novices rarely recognize the original need for the edit and I've seen it happen many times where the votes abruptly stop coming in once someone has added the edit line.
I could go on about the content and format of your answer, but I don't want to risk giving too much advice that isn't always true. The next thing that influences number of votes is the time of day that a question appears on the active page. We all live in different time zones, but a majority of our users are in North America. If you do your edits or post your answers during daylight/business hours for North America, you will get more instantaneous views and a higher chance of fast upvotes, which gets you onto the "hot topics" list. However, it also means that you will be off the active front page relatively quickly. There's a trade off there. In the reverse time slot, you'll stay on the active page longer, but you'll definitely get less views. In your case, you might have hit a sweet spot time-wise.
There is much more that goes into how many upvotes an answer gets. And what makes vote-hunting hardest is that these factors have varying impacts on number of upvotes such that it is very difficult to predict whether a question will be a hit or a miss. It's more of an experience-based instinct that some users will have.
