The artboard is transparent. You need to add a white object below or turn off the transparency usage.
You exported a PNG. They can be transparent. Your image viewer shows a dark grey background where there is nothing below. Do you need the image to be transparent, because you want it on different backgrounds? In that case leave it like this and put it on whatever background you like. Otherwise turn off the transparency when exporting in the Export options (click on the cogwheel icon to the right edge of the dialog)
Monika Gause, thanks for the detail explanation.
In the end of your video you are opening the PNG in an image viewer - I don't know which one it is.
Thanks for all.