Thanks.
I'm glad you like the interface. I've spent hours think about it and working on it. And you can look forward to more improvements in the next few weeks and months.
Unfortunately the XML export seems to have been replaced by a "ubox" binary file format.
.ubox files are just zips files with a .ubox extension/
Rename the .ubox to .zip and then open it with your favorite zip extractor. Inside is the standard .xml file that you're used to.
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A couple other points:
You can now just double click on .ubox file to open it with Universe Sandbox.
You can also drag .ubox or .xml files into the Universe Sandbox window to open them.
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Over the next few months I'll be adding more improvements to the .ubox file format including saving particle (dust) data and custom textures and tutorials.