x.xml

<?xml version="1.0" encoding="UTF-8"?>
<hello xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:noNamespaceSchemaLocation="https://raw.github.com/gist/eca35d6fc42ccb20906c/303e22e0a851f962cf381ae6fac4cac71d138a6f/x.xsd">
    <universe />
</hello>

x.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
  <xs:element name="hello">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="universe"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="universe">
    <xs:complexType/>
  </xs:element>
</xs:schema>