Add this to any mesh to flip the normals. Useful to create 360 spheres, etc. Run once and remove the script.

UnityFlipNormals

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class FlipNormals : MonoBehaviour {

 // Use this for initialization
 void Start () {
	Mesh mesh = this.GetComponent<MeshFilter>().sharedMesh;
	Vector3[] normals = mesh.normals;
	for(int i=0;i<normals.Length;i++)
	{
		normals[i] = -1 * normals[i];
	}
	mesh.normals = normals;
	for(int i=0;i<mesh.subMeshCount;i++)
	{
		int[] tris = mesh.GetTriangles(i);
		for(int j =0;j<tris.Length;j+=3)
		{
			//swap order of tris vertices 
			int temp = tris[j];
			tris[j] = tris[j+1];
			tris[j + 1] = temp;
		}
		mesh.SetTriangles(tris, i);
	}
 }
}