Mirror Modifier Not Working in Blender
Gap in the middle or doubled geometry
Also known as: Blender mirror modifier gap · Blender mirror modifier wrong · Blender symmetry not working · Blender mirror clipping · Blender mirror modifier center
Quick answer
A Mirror modifier shows a gap or doubled geometry in the middle when the object's origin is not on the mirror axis or the center vertices are not exactly on it. Set the origin to the center, place the center vertices at zero on the mirror axis, and turn on Clipping and Merge in the modifier so vertices on the axis stay stuck together while you edit.
How to fix it
Move the 3D cursor to the world origin with Shift+C, then use Object > Set Origin > Origin to 3D Cursor, and apply the scale so the mirror works as expected. If one side is the wrong way round, check the axis settings and any negative scale on the object. When the model is finished, apply the modifier, merge the center vertices and check the normals before baking or exporting.