Why doesn't gazebo publish /odom topic?

edit

This is my launch file:

<launch>

  <param name="use_sim_time" value="true" />

  <param name="robot_description" command="$(find xacro)/xacro '$(find genius_description)/urdf/test.xacro'"/>

  <node name="state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"/>

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find genius_gazebo)/worlds/test.world" />
  </include>

  <node name="genius_spawn" pkg="gazebo_ros" type="spawn_model" output="screen"
   args="-urdf -param robot_description -model genius" />

   <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"
         respawn="false" output="screen">
  </node>
   <node pkg="rviz" type="rviz" name="rviz"/>

</launch>

So as you can see, I am spawning gazebo but at the end I do not see /odom topic to be published. This is strange because this launch file worked earlier and now, no odom topic. Shouldn't gazebo publish /odom topic automatically?

Here is the gazebo plugin I am using:

<gazebo>
    <plugin name="differential_drive_controller" filename="libgazebo_ros_diff_drive.so">
      <alwaysOn>true</alwaysOn>
      <updateRate>30</updateRate>
          <legacyMode>false</legacyMode>
      <leftJoint>left_wheel_hinge</leftJoint>
      <rightJoint>right_wheel_hinge</rightJoint>
      <wheelSeparation>${chassisWidth+wheelWidth}</wheelSeparation>
      <wheelDiameter>${2*wheelRadius}</wheelDiameter>
      <torque>10</torque>
      <commandTopic>cmd_vel</commandTopic>
      <odometryTopic>odom</odometryTopic>
      <odometryFrame>odom</odometryFrame>
      <robotBaseFrame>base_link</robotBaseFrame>
      <rosDebugLevel>1</rosDebugLevel>
      <publishWheelTF>true</publishWheelTF>
      <publishWheelJointState>true</publishWheelJointState>
      <wheelTorque>20</wheelTorque>
    </plugin>
  </gazebo>