How to make User Objects for Universe Sandbox 2?

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I've looked all over the Internet for a tutorial, but came up with zero results.
In the "Add" menu of Universe Sandbox 2, there is a button named "User Objects".
This obviously means there is a way to add your own objects into the game. What I don't know however, is how to do this.
There is a tab next to it called "Objects" containing things like a Dice and a Baseball.
How would I go about making my own?
Many thanks.
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How do you create a custom Object in Universe Sandbox 2?
Universe Sandbox 2 is mostly realistic. Its accuracy degrades as you increase the flow of time. The slower the time, the more accurate the calculations. This means that if you want to simulate the Solar System over a long period of time, it won't be very accurate.Making custom objects #1, Universe Sandbox 2
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